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    #include <linux/kernel.h>
    #include <linux/errno.h>
    #include <linux/err.h>
    #include <linux/spinlock.h>
    
    #include <linux/mm.h>
    #include <linux/memremap.h>
    #include <linux/pagemap.h>
    #include <linux/rmap.h>
    #include <linux/swap.h>
    #include <linux/swapops.h>
    
    #include <linux/sched.h>
    #include <linux/rwsem.h>
    #include <linux/hugetlb.h>
    
    #include <asm/mmu_context.h>
    #include <asm/pgtable.h>
    #include <asm/tlbflush.h>
    
    #include "internal.h"
    
    static struct page *no_page_table(struct vm_area_struct *vma,
    		unsigned int flags)
    {
    	/*
    	 * When core dumping an enormous anonymous area that nobody
    	 * has touched so far, we don't want to allocate unnecessary pages or
    	 * page tables.  Return error instead of NULL to skip handle_mm_fault,
    	 * then get_dump_page() will return NULL to leave a hole in the dump.
    	 * But we can only make this optimization where a hole would surely
    	 * be zero-filled if handle_mm_fault() actually did handle it.
    	 */
    	if ((flags & FOLL_DUMP) && (!vma->vm_ops || !vma->vm_ops->fault))
    		return ERR_PTR(-EFAULT);
    	return NULL;
    }
    
    static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
    		pte_t *pte, unsigned int flags)
    {
    	/* No page to get reference */
    	if (flags & FOLL_GET)
    		return -EFAULT;
    
    	if (flags & FOLL_TOUCH) {
    		pte_t entry = *pte;
    
    		if (flags & FOLL_WRITE)
    			entry = pte_mkdirty(entry);
    		entry = pte_mkyoung(entry);
    
    		if (!pte_same(*pte, entry)) {
    			set_pte_at(vma->vm_mm, address, pte, entry);
    			update_mmu_cache(vma, address, pte);
    		}
    	}
    
    	/* Proper page table entry exists, but no corresponding struct page */
    	return -EEXIST;
    }
    
    /*
     * FOLL_FORCE can write to even unwritable pte's, but only
     * after we've gone through a COW cycle and they are dirty.
     */
    static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
    {
    	return pte_write(pte) ||
    		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
    }
    
    static struct page *follow_page_pte(struct vm_area_struct *vma,
    		unsigned long address, pmd_t *pmd, unsigned int flags)
    {
    	struct mm_struct *mm = vma->vm_mm;
    	struct dev_pagemap *pgmap = NULL;
    	struct page *page;
    	spinlock_t *ptl;
    	pte_t *ptep, pte;
    
    retry:
    	if (unlikely(pmd_bad(*pmd)))
    		return no_page_table(vma, flags);
    
    	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
    	pte = *ptep;
    	if (!pte_present(pte)) {
    		swp_entry_t entry;
    		/*
    		 * KSM's break_ksm() relies upon recognizing a ksm page
    		 * even while it is being migrated, so for that case we
    		 * need migration_entry_wait().
    		 */
    		if (likely(!(flags & FOLL_MIGRATION)))
    			goto no_page;
    		if (pte_none(pte))
    			goto no_page;
    		entry = pte_to_swp_entry(pte);
    		if (!is_migration_entry(entry))
    			goto no_page;
    		pte_unmap_unlock(ptep, ptl);
    		migration_entry_wait(mm, pmd, address);
    		goto retry;
    	}
    	if ((flags & FOLL_NUMA) && pte_protnone(pte))
    		goto no_page;
    	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
    		pte_unmap_unlock(ptep, ptl);
    		return NULL;
    	}
    
    	page = vm_normal_page(vma, address, pte);
    	if (!page && pte_devmap(pte) && (flags & FOLL_GET)) {
    		/*
    		 * Only return device mapping pages in the FOLL_GET case since
    		 * they are only valid while holding the pgmap reference.
    		 */
    		pgmap = get_dev_pagemap(pte_pfn(pte), NULL);
    		if (pgmap)
    			page = pte_page(pte);
    		else
    			goto no_page;
    	} else if (unlikely(!page)) {
    		if (flags & FOLL_DUMP) {
    			/* Avoid special (like zero) pages in core dumps */
    			page = ERR_PTR(-EFAULT);
    			goto out;
    		}
    
    		if (is_zero_pfn(pte_pfn(pte))) {
    			page = pte_page(pte);
    		} else {
    			int ret;
    
    			ret = follow_pfn_pte(vma, address, ptep, flags);
    			page = ERR_PTR(ret);
    			goto out;
    		}
    	}
    
    	if (flags & FOLL_SPLIT && PageTransCompound(page)) {
    		int ret;
    		get_page(page);
    		pte_unmap_unlock(ptep, ptl);
    		lock_page(page);
    		ret = split_huge_page(page);
    		unlock_page(page);
    		put_page(page);
    		if (ret)
    			return ERR_PTR(ret);
    		goto retry;
    	}
    
    	if (flags & FOLL_GET) {
    		get_page(page);
    
    		/* drop the pgmap reference now that we hold the page */
    		if (pgmap) {
    			put_dev_pagemap(pgmap);
    			pgmap = NULL;
    		}
    	}
    	if (flags & FOLL_TOUCH) {
    		if ((flags & FOLL_WRITE) &&
    		    !pte_dirty(pte) && !PageDirty(page))
    			set_page_dirty(page);
    		/*
    		 * pte_mkyoung() would be more correct here, but atomic care
    		 * is needed to avoid losing the dirty bit: it is easier to use
    		 * mark_page_accessed().
    		 */
    		mark_page_accessed(page);
    	}
    	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
    		/* Do not mlock pte-mapped THP */
    		if (PageTransCompound(page))
    			goto out;
    
    		/*
    		 * The preliminary mapping check is mainly to avoid the
    		 * pointless overhead of lock_page on the ZERO_PAGE
    		 * which might bounce very badly if there is contention.
    		 *
    		 * If the page is already locked, we don't need to
    		 * handle it now - vmscan will handle it later if and
    		 * when it attempts to reclaim the page.
    		 */
    		if (page->mapping && trylock_page(page)) {
    			lru_add_drain();  /* push cached pages to LRU */
    			/*
    			 * Because we lock page here, and migration is
    			 * blocked by the pte's page reference, and we
    			 * know the page is still mapped, we don't even
    			 * need to check for file-cache page truncation.
    			 */
    			mlock_vma_page(page);
    			unlock_page(page);
    		}
    	}
    out:
    	pte_unmap_unlock(ptep, ptl);
    	return page;
    no_page:
    	pte_unmap_unlock(ptep, ptl);
    	if (!pte_none(pte))
    		return NULL;
    	return no_page_table(vma, flags);
    }
    
    /**
     * follow_page_mask - look up a page descriptor from a user-virtual address
     * @vma: vm_area_struct mapping @address
     * @address: virtual address to look up
     * @flags: flags modifying lookup behaviour
     * @page_mask: on output, *page_mask is set according to the size of the page
     *
     * @flags can have FOLL_ flags set, defined in <linux/mm.h>
     *
     * Returns the mapped (struct page *), %NULL if no mapping exists, or
     * an error pointer if there is a mapping to something not represented
     * by a page descriptor (see also vm_normal_page()).
     */
    struct page *follow_page_mask(struct vm_area_struct *vma,
    			      unsigned long address, unsigned int flags,
    			      unsigned int *page_mask)
    {
    	pgd_t *pgd;
    	pud_t *pud;
    	pmd_t *pmd;
    	spinlock_t *ptl;
    	struct page *page;
    	struct mm_struct *mm = vma->vm_mm;
    
    	*page_mask = 0;
    
    	page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
    	if (!IS_ERR(page)) {
    		BUG_ON(flags & FOLL_GET);
    		return page;
    	}
    
    	pgd = pgd_offset(mm, address);
    	if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
    		return no_page_table(vma, flags);
    
    	pud = pud_offset(pgd, address);
    	if (pud_none(*pud))
    		return no_page_table(vma, flags);
    	if (pud_huge(*pud) && vma->vm_flags & VM_HUGETLB) {
    		page = follow_huge_pud(mm, address, pud, flags);
    		if (page)
    			return page;
    		return no_page_table(vma, flags);
    	}
    	if (unlikely(pud_bad(*pud)))
    		return no_page_table(vma, flags);
    
    	pmd = pmd_offset(pud, address);
    	if (pmd_none(*pmd))
    		return no_page_table(vma, flags);
    	if (pmd_huge(*pmd) && vma->vm_flags & VM_HUGETLB) {
    		page = follow_huge_pmd(mm, address, pmd, flags);
    		if (page)
    			return page;
    		return no_page_table(vma, flags);
    	}
    	if ((flags & FOLL_NUMA) && pmd_protnone(*pmd))
    		return no_page_table(vma, flags);
    	if (pmd_devmap(*pmd)) {
    		ptl = pmd_lock(mm, pmd);
    		page = follow_devmap_pmd(vma, address, pmd, flags);
    		spin_unlock(ptl);
    		if (page)
    			return page;
    	}
    	if (likely(!pmd_trans_huge(*pmd)))
    		return follow_page_pte(vma, address, pmd, flags);
    
    	ptl = pmd_lock(mm, pmd);
    	if (unlikely(!pmd_trans_huge(*pmd))) {
    		spin_unlock(ptl);
    		return follow_page_pte(vma, address, pmd, flags);
    	}
    	if (flags & FOLL_SPLIT) {
    		int ret;
    		page = pmd_page(*pmd);
    		if (is_huge_zero_page(page)) {
    			spin_unlock(ptl);
    			ret = 0;
    			split_huge_pmd(vma, pmd, address);
    			if (pmd_trans_unstable(pmd))
    				ret = -EBUSY;
    		} else {
    			get_page(page);
    			spin_unlock(ptl);
    			lock_page(page);
    			ret = split_huge_page(page);
    			unlock_page(page);
    			put_page(page);
    			if (pmd_none(*pmd))
    				return no_page_table(vma, flags);
    		}
    
    		return ret ? ERR_PTR(ret) :
    			follow_page_pte(vma, address, pmd, flags);
    	}
    
    	page = follow_trans_huge_pmd(vma, address, pmd, flags);
    	spin_unlock(ptl);
    	*page_mask = HPAGE_PMD_NR - 1;
    	return page;
    }
    
    static int get_gate_page(struct mm_struct *mm, unsigned long address,
    		unsigned int gup_flags, struct vm_area_struct **vma,
    		struct page **page)
    {
    	pgd_t *pgd;
    	pud_t *pud;
    	pmd_t *pmd;
    	pte_t *pte;
    	int ret = -EFAULT;
    
    	/* user gate pages are read-only */
    	if (gup_flags & FOLL_WRITE)
    		return -EFAULT;
    	if (address > TASK_SIZE)
    		pgd = pgd_offset_k(address);
    	else
    		pgd = pgd_offset_gate(mm, address);
    	BUG_ON(pgd_none(*pgd));
    	pud = pud_offset(pgd, address);
    	BUG_ON(pud_none(*pud));
    	pmd = pmd_offset(pud, address);
    	if (pmd_none(*pmd))
    		return -EFAULT;
    	VM_BUG_ON(pmd_trans_huge(*pmd));
    	pte = pte_offset_map(pmd, address);
    	if (pte_none(*pte))
    		goto unmap;
    	*vma = get_gate_vma(mm);
    	if (!page)
    		goto out;
    	*page = vm_normal_page(*vma, address, *pte);
    	if (!*page) {
    		if ((gup_flags & FOLL_DUMP) || !is_zero_pfn(pte_pfn(*pte)))
    			goto unmap;
    		*page = pte_page(*pte);
    	}
    	get_page(*page);
    out:
    	ret = 0;
    unmap:
    	pte_unmap(pte);
    	return ret;
    }
    
    /*
     * mmap_sem must be held on entry.  If @nonblocking != NULL and
     * *@flags does not include FOLL_NOWAIT, the mmap_sem may be released.
     * If it is, *@nonblocking will be set to 0 and -EBUSY returned.
     */
    static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
    		unsigned long address, unsigned int *flags, int *nonblocking)
    {
    	unsigned int fault_flags = 0;
    	int ret;
    
    	/* mlock all present pages, but do not fault in new pages */
    	if ((*flags & (FOLL_POPULATE | FOLL_MLOCK)) == FOLL_MLOCK)
    		return -ENOENT;
    	if (*flags & FOLL_WRITE)
    		fault_flags |= FAULT_FLAG_WRITE;
    	if (*flags & FOLL_REMOTE)
    		fault_flags |= FAULT_FLAG_REMOTE;
    	if (nonblocking)
    		fault_flags |= FAULT_FLAG_ALLOW_RETRY;
    	if (*flags & FOLL_NOWAIT)
    		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
    	if (*flags & FOLL_TRIED) {
    		VM_WARN_ON_ONCE(fault_flags & FAULT_FLAG_ALLOW_RETRY);
    		fault_flags |= FAULT_FLAG_TRIED;
    	}
    
    	ret = handle_mm_fault(vma, address, fault_flags);
    	if (ret & VM_FAULT_ERROR) {
    		if (ret & VM_FAULT_OOM)
    			return -ENOMEM;
    		if (ret & (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE))
    			return *flags & FOLL_HWPOISON ? -EHWPOISON : -EFAULT;
    		if (ret & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV))
    			return -EFAULT;
    		BUG();
    	}
    
    	if (tsk) {
    		if (ret & VM_FAULT_MAJOR)
    			tsk->maj_flt++;
    		else
    			tsk->min_flt++;
    	}
    
    	if (ret & VM_FAULT_RETRY) {
    		if (nonblocking)
    			*nonblocking = 0;
    		return -EBUSY;
    	}
    
    	/*
    	 * The VM_FAULT_WRITE bit tells us that do_wp_page has broken COW when
    	 * necessary, even if maybe_mkwrite decided not to set pte_write. We
    	 * can thus safely do subsequent page lookups as if they were reads.
    	 * But only do so when looping for pte_write is futile: in some cases
    	 * userspace may also be wanting to write to the gotten user page,
    	 * which a read fault here might prevent (a readonly page might get
    	 * reCOWed by userspace write).
    	 */
    	if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
    	        *flags |= FOLL_COW;
    	return 0;
    }
    
    static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
    {
    	vm_flags_t vm_flags = vma->vm_flags;
    	int write = (gup_flags & FOLL_WRITE);
    	int foreign = (gup_flags & FOLL_REMOTE);
    
    	if (vm_flags & (VM_IO | VM_PFNMAP))
    		return -EFAULT;
    
    	if (write) {
    		if (!(vm_flags & VM_WRITE)) {
    			if (!(gup_flags & FOLL_FORCE))
    				return -EFAULT;
    			/*
    			 * We used to let the write,force case do COW in a
    			 * VM_MAYWRITE VM_SHARED !VM_WRITE vma, so ptrace could
    			 * set a breakpoint in a read-only mapping of an
    			 * executable, without corrupting the file (yet only
    			 * when that file had been opened for writing!).
    			 * Anon pages in shared mappings are surprising: now
    			 * just reject it.
    			 */
    			if (!is_cow_mapping(vm_flags))
    				return -EFAULT;
    		}
    	} else if (!(vm_flags & VM_READ)) {
    		if (!(gup_flags & FOLL_FORCE))
    			return -EFAULT;
    		/*
    		 * Is there actually any vma we can reach here which does not
    		 * have VM_MAYREAD set?
    		 */
    		if (!(vm_flags & VM_MAYREAD))
    			return -EFAULT;
    	}
    	/*
    	 * gups are always data accesses, not instruction
    	 * fetches, so execute=false here
    	 */
    	if (!arch_vma_access_permitted(vma, write, false, foreign))
    		return -EFAULT;
    	return 0;
    }
    
    /**
     * __get_user_pages() - pin user pages in memory
     * @tsk:	task_struct of target task
     * @mm:		mm_struct of target mm
     * @start:	starting user address
     * @nr_pages:	number of pages from start to pin
     * @gup_flags:	flags modifying pin behaviour
     * @pages:	array that receives pointers to the pages pinned.
     *		Should be at least nr_pages long. Or NULL, if caller
     *		only intends to ensure the pages are faulted in.
     * @vmas:	array of pointers to vmas corresponding to each page.
     *		Or NULL if the caller does not require them.
     * @nonblocking: whether waiting for disk IO or mmap_sem contention
     *
     * Returns number of pages pinned. This may be fewer than the number
     * requested. If nr_pages is 0 or negative, returns 0. If no pages
     * were pinned, returns -errno. Each page returned must be released
     * with a put_page() call when it is finished with. vmas will only
     * remain valid while mmap_sem is held.
     *
     * Must be called with mmap_sem held.  It may be released.  See below.
     *
     * __get_user_pages walks a process's page tables and takes a reference to
     * each struct page that each user address corresponds to at a given
     * instant. That is, it takes the page that would be accessed if a user
     * thread accesses the given user virtual address at that instant.
     *
     * This does not guarantee that the page exists in the user mappings when
     * __get_user_pages returns, and there may even be a completely different
     * page there in some cases (eg. if mmapped pagecache has been invalidated
     * and subsequently re faulted). However it does guarantee that the page
     * won't be freed completely. And mostly callers simply care that the page
     * contains data that was valid *at some point in time*. Typically, an IO
     * or similar operation cannot guarantee anything stronger anyway because
     * locks can't be held over the syscall boundary.
     *
     * If @gup_flags & FOLL_WRITE == 0, the page must not be written to. If
     * the page is written to, set_page_dirty (or set_page_dirty_lock, as
     * appropriate) must be called after the page is finished with, and
     * before put_page is called.
     *
     * If @nonblocking != NULL, __get_user_pages will not wait for disk IO
     * or mmap_sem contention, and if waiting is needed to pin all pages,
     * *@nonblocking will be set to 0.  Further, if @gup_flags does not
     * include FOLL_NOWAIT, the mmap_sem will be released via up_read() in
     * this case.
     *
     * A caller using such a combination of @nonblocking and @gup_flags
     * must therefore hold the mmap_sem for reading only, and recognize
     * when it's been released.  Otherwise, it must be held for either
     * reading or writing and will not be released.
     *
     * In most cases, get_user_pages or get_user_pages_fast should be used
     * instead of __get_user_pages. __get_user_pages should be used only if
     * you need some special @gup_flags.
     */
    static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
    		unsigned long start, unsigned long nr_pages,
    		unsigned int gup_flags, struct page **pages,
    		struct vm_area_struct **vmas, int *nonblocking)
    {
    	long i = 0;
    	unsigned int page_mask;
    	struct vm_area_struct *vma = NULL;
    
    	if (!nr_pages)
    		return 0;
    
    	VM_BUG_ON(!!pages != !!(gup_flags & FOLL_GET));
    
    	/*
    	 * If FOLL_FORCE is set then do not force a full fault as the hinting
    	 * fault information is unrelated to the reference behaviour of a task
    	 * using the address space
    	 */
    	if (!(gup_flags & FOLL_FORCE))
    		gup_flags |= FOLL_NUMA;
    
    	do {
    		struct page *page;
    		unsigned int foll_flags = gup_flags;
    		unsigned int page_increm;
    
    		/* first iteration or cross vma bound */
    		if (!vma || start >= vma->vm_end) {
    			vma = find_extend_vma(mm, start);
    			if (!vma && in_gate_area(mm, start)) {
    				int ret;
    				ret = get_gate_page(mm, start & PAGE_MASK,
    						gup_flags, &vma,
    						pages ? &pages[i] : NULL);
    				if (ret)
    					return i ? : ret;
    				page_mask = 0;
    				goto next_page;
    			}
    
    			if (!vma || check_vma_flags(vma, gup_flags))
    				return i ? : -EFAULT;
    			if (is_vm_hugetlb_page(vma)) {
    				i = follow_hugetlb_page(mm, vma, pages, vmas,
    						&start, &nr_pages, i,
    						gup_flags);
    				continue;
    			}
    		}
    retry:
    		/*
    		 * If we have a pending SIGKILL, don't keep faulting pages and
    		 * potentially allocating memory.
    		 */
    		if (unlikely(fatal_signal_pending(current)))
    			return i ? i : -ERESTARTSYS;
    		cond_resched();
    		page = follow_page_mask(vma, start, foll_flags, &page_mask);
    		if (!page) {
    			int ret;
    			ret = faultin_page(tsk, vma, start, &foll_flags,
    					nonblocking);
    			switch (ret) {
    			case 0:
    				goto retry;
    			case -EFAULT:
    			case -ENOMEM:
    			case -EHWPOISON:
    				return i ? i : ret;
    			case -EBUSY:
    				return i;
    			case -ENOENT:
    				goto next_page;
    			}
    			BUG();
    		} else if (PTR_ERR(page) == -EEXIST) {
    			/*
    			 * Proper page table entry exists, but no corresponding
    			 * struct page.
    			 */
    			goto next_page;
    		} else if (IS_ERR(page)) {
    			return i ? i : PTR_ERR(page);
    		}
    		if (pages) {
    			pages[i] = page;
    			flush_anon_page(vma, page, start);
    			flush_dcache_page(page);
    			page_mask = 0;
    		}
    next_page:
    		if (vmas) {
    			vmas[i] = vma;
    			page_mask = 0;
    		}
    		page_increm = 1 + (~(start >> PAGE_SHIFT) & page_mask);
    		if (page_increm > nr_pages)
    			page_increm = nr_pages;
    		i += page_increm;
    		start += page_increm * PAGE_SIZE;
    		nr_pages -= page_increm;
    	} while (nr_pages);
    	return i;
    }
    
    bool vma_permits_fault(struct vm_area_struct *vma, unsigned int fault_flags)
    {
    	bool write   = !!(fault_flags & FAULT_FLAG_WRITE);
    	bool foreign = !!(fault_flags & FAULT_FLAG_REMOTE);
    	vm_flags_t vm_flags = write ? VM_WRITE : VM_READ;
    
    	if (!(vm_flags & vma->vm_flags))
    		return false;
    
    	/*
    	 * The architecture might have a hardware protection
    	 * mechanism other than read/write that can deny access.
    	 *
    	 * gup always represents data access, not instruction
    	 * fetches, so execute=false here:
    	 */
    	if (!arch_vma_access_permitted(vma, write, false, foreign))
    		return false;
    
    	return true;
    }
    
    /*
     * fixup_user_fault() - manually resolve a user page fault
     * @tsk:	the task_struct to use for page fault accounting, or
     *		NULL if faults are not to be recorded.
     * @mm:		mm_struct of target mm
     * @address:	user address
     * @fault_flags:flags to pass down to handle_mm_fault()
     * @unlocked:	did we unlock the mmap_sem while retrying, maybe NULL if caller
     *		does not allow retry
     *
     * This is meant to be called in the specific scenario where for locking reasons
     * we try to access user memory in atomic context (within a pagefault_disable()
     * section), this returns -EFAULT, and we want to resolve the user fault before
     * trying again.
     *
     * Typically this is meant to be used by the futex code.
     *
     * The main difference with get_user_pages() is that this function will
     * unconditionally call handle_mm_fault() which will in turn perform all the
     * necessary SW fixup of the dirty and young bits in the PTE, while
     * get_user_pages() only guarantees to update these in the struct page.
     *
     * This is important for some architectures where those bits also gate the
     * access permission to the page because they are maintained in software.  On
     * such architectures, gup() will not be enough to make a subsequent access
     * succeed.
     *
     * This function will not return with an unlocked mmap_sem. So it has not the
     * same semantics wrt the @mm->mmap_sem as does filemap_fault().
     */
    int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
    		     unsigned long address, unsigned int fault_flags,
    		     bool *unlocked)
    {
    	struct vm_area_struct *vma;
    	int ret, major = 0;
    
    	if (unlocked)
    		fault_flags |= FAULT_FLAG_ALLOW_RETRY;
    
    retry:
    	vma = find_extend_vma(mm, address);
    	if (!vma || address < vma->vm_start)
    		return -EFAULT;
    
    	if (!vma_permits_fault(vma, fault_flags))
    		return -EFAULT;
    
    	ret = handle_mm_fault(vma, address, fault_flags);
    	major |= ret & VM_FAULT_MAJOR;
    	if (ret & VM_FAULT_ERROR) {
    		if (ret & VM_FAULT_OOM)
    			return -ENOMEM;
    		if (ret & (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE))
    			return -EHWPOISON;
    		if (ret & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV))
    			return -EFAULT;
    		BUG();
    	}
    
    	if (ret & VM_FAULT_RETRY) {
    		down_read(&mm->mmap_sem);
    		if (!(fault_flags & FAULT_FLAG_TRIED)) {
    			*unlocked = true;
    			fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
    			fault_flags |= FAULT_FLAG_TRIED;
    			goto retry;
    		}
    	}
    
    	if (tsk) {
    		if (major)
    			tsk->maj_flt++;
    		else
    			tsk->min_flt++;
    	}
    	return 0;
    }
    EXPORT_SYMBOL_GPL(fixup_user_fault);
    
    static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
    						struct mm_struct *mm,
    						unsigned long start,
    						unsigned long nr_pages,
    						struct page **pages,
    						struct vm_area_struct **vmas,
    						int *locked, bool notify_drop,
    						unsigned int flags)
    {
    	long ret, pages_done;
    	bool lock_dropped;
    
    	if (locked) {
    		/* if VM_FAULT_RETRY can be returned, vmas become invalid */
    		BUG_ON(vmas);
    		/* check caller initialized locked */
    		BUG_ON(*locked != 1);
    	}
    
    	if (pages)
    		flags |= FOLL_GET;
    
    	pages_done = 0;
    	lock_dropped = false;
    	for (;;) {
    		ret = __get_user_pages(tsk, mm, start, nr_pages, flags, pages,
    				       vmas, locked);
    		if (!locked)
    			/* VM_FAULT_RETRY couldn't trigger, bypass */
    			return ret;
    
    		/* VM_FAULT_RETRY cannot return errors */
    		if (!*locked) {
    			BUG_ON(ret < 0);
    			BUG_ON(ret >= nr_pages);
    		}
    
    		if (!pages)
    			/* If it's a prefault don't insist harder */
    			return ret;
    
    		if (ret > 0) {
    			nr_pages -= ret;
    			pages_done += ret;
    			if (!nr_pages)
    				break;
    		}
    		if (*locked) {
    			/* VM_FAULT_RETRY didn't trigger */
    			if (!pages_done)
    				pages_done = ret;
    			break;
    		}
    		/* VM_FAULT_RETRY triggered, so seek to the faulting offset */
    		pages += ret;
    		start += ret << PAGE_SHIFT;
    
    		/*
    		 * Repeat on the address that fired VM_FAULT_RETRY
    		 * without FAULT_FLAG_ALLOW_RETRY but with
    		 * FAULT_FLAG_TRIED.
    		 */
    		*locked = 1;
    		lock_dropped = true;
    		down_read(&mm->mmap_sem);
    		ret = __get_user_pages(tsk, mm, start, 1, flags | FOLL_TRIED,
    				       pages, NULL, NULL);
    		if (ret != 1) {
    			BUG_ON(ret > 1);
    			if (!pages_done)
    				pages_done = ret;
    			break;
    		}
    		nr_pages--;
    		pages_done++;
    		if (!nr_pages)
    			break;
    		pages++;
    		start += PAGE_SIZE;
    	}
    	if (notify_drop && lock_dropped && *locked) {
    		/*
    		 * We must let the caller know we temporarily dropped the lock
    		 * and so the critical section protected by it was lost.
    		 */
    		up_read(&mm->mmap_sem);
    		*locked = 0;
    	}
    	return pages_done;
    }
    
    /*
     * We can leverage the VM_FAULT_RETRY functionality in the page fault
     * paths better by using either get_user_pages_locked() or
     * get_user_pages_unlocked().
     *
     * get_user_pages_locked() is suitable to replace the form:
     *
     *      down_read(&mm->mmap_sem);
     *      do_something()
     *      get_user_pages(tsk, mm, ..., pages, NULL);
     *      up_read(&mm->mmap_sem);
     *
     *  to:
     *
     *      int locked = 1;
     *      down_read(&mm->mmap_sem);
     *      do_something()
     *      get_user_pages_locked(tsk, mm, ..., pages, &locked);
     *      if (locked)
     *          up_read(&mm->mmap_sem);
     */
    long get_user_pages_locked(unsigned long start, unsigned long nr_pages,
    			   unsigned int gup_flags, struct page **pages,
    			   int *locked)
    {
    	return __get_user_pages_locked(current, current->mm, start, nr_pages,
    				       pages, NULL, locked, true,
    				       gup_flags | FOLL_TOUCH);
    }
    EXPORT_SYMBOL(get_user_pages_locked);
    
    /*
     * Same as get_user_pages_unlocked(...., FOLL_TOUCH) but it allows to
     * pass additional gup_flags as last parameter (like FOLL_HWPOISON).
     *
     * NOTE: here FOLL_TOUCH is not set implicitly and must be set by the
     * caller if required (just like with __get_user_pages). "FOLL_GET",
     * "FOLL_WRITE" and "FOLL_FORCE" are set implicitly as needed
     * according to the parameters "pages", "write", "force"
     * respectively.
     */
    __always_inline long __get_user_pages_unlocked(struct task_struct *tsk, struct mm_struct *mm,
    					       unsigned long start, unsigned long nr_pages,
    					       struct page **pages, unsigned int gup_flags)
    {
    	long ret;
    	int locked = 1;
    
    	down_read(&mm->mmap_sem);
    	ret = __get_user_pages_locked(tsk, mm, start, nr_pages, pages, NULL,
    				      &locked, false, gup_flags);
    	if (locked)
    		up_read(&mm->mmap_sem);
    	return ret;
    }
    EXPORT_SYMBOL(__get_user_pages_unlocked);
    
    /*
     * get_user_pages_unlocked() is suitable to replace the form:
     *
     *      down_read(&mm->mmap_sem);
     *      get_user_pages(tsk, mm, ..., pages, NULL);
     *      up_read(&mm->mmap_sem);
     *
     *  with:
     *
     *      get_user_pages_unlocked(tsk, mm, ..., pages);
     *
     * It is functionally equivalent to get_user_pages_fast so
     * get_user_pages_fast should be used instead, if the two parameters
     * "tsk" and "mm" are respectively equal to current and current->mm,
     * or if "force" shall be set to 1 (get_user_pages_fast misses the
     * "force" parameter).
     */
    long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
    			     struct page **pages, unsigned int gup_flags)
    {
    	return __get_user_pages_unlocked(current, current->mm, start, nr_pages,
    					 pages, gup_flags | FOLL_TOUCH);
    }
    EXPORT_SYMBOL(get_user_pages_unlocked);
    
    /*
     * get_user_pages_remote() - pin user pages in memory
     * @tsk:	the task_struct to use for page fault accounting, or
     *		NULL if faults are not to be recorded.
     * @mm:		mm_struct of target mm
     * @start:	starting user address
     * @nr_pages:	number of pages from start to pin
     * @gup_flags:	flags modifying lookup behaviour
     * @pages:	array that receives pointers to the pages pinned.
     *		Should be at least nr_pages long. Or NULL, if caller
     *		only intends to ensure the pages are faulted in.
     * @vmas:	array of pointers to vmas corresponding to each page.
     *		Or NULL if the caller does not require them.
     *
     * Returns number of pages pinned. This may be fewer than the number
     * requested. If nr_pages is 0 or negative, returns 0. If no pages
     * were pinned, returns -errno. Each page returned must be released
     * with a put_page() call when it is finished with. vmas will only
     * remain valid while mmap_sem is held.
     *
     * Must be called with mmap_sem held for read or write.
     *
     * get_user_pages walks a process's page tables and takes a reference to
     * each struct page that each user address corresponds to at a given
     * instant. That is, it takes the page that would be accessed if a user
     * thread accesses the given user virtual address at that instant.
     *
     * This does not guarantee that the page exists in the user mappings when
     * get_user_pages returns, and there may even be a completely different
     * page there in some cases (eg. if mmapped pagecache has been invalidated
     * and subsequently re faulted). However it does guarantee that the page
     * won't be freed completely. And mostly callers simply care that the page
     * contains data that was valid *at some point in time*. Typically, an IO
     * or similar operation cannot guarantee anything stronger anyway because
     * locks can't be held over the syscall boundary.
     *
     * If gup_flags & FOLL_WRITE == 0, the page must not be written to. If the page
     * is written to, set_page_dirty (or set_page_dirty_lock, as appropriate) must
     * be called after the page is finished with, and before put_page is called.
     *
     * get_user_pages is typically used for fewer-copy IO operations, to get a
     * handle on the memory by some means other than accesses via the user virtual
     * addresses. The pages may be submitted for DMA to devices or accessed via
     * their kernel linear mapping (via the kmap APIs). Care should be taken to
     * use the correct cache flushing APIs.
     *
     * See also get_user_pages_fast, for performance critical applications.
     *
     * get_user_pages should be phased out in favor of
     * get_user_pages_locked|unlocked or get_user_pages_fast. Nothing
     * should use get_user_pages because it cannot pass
     * FAULT_FLAG_ALLOW_RETRY to handle_mm_fault.
     */
    long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
    		unsigned long start, unsigned long nr_pages,
    		unsigned int gup_flags, struct page **pages,
    		struct vm_area_struct **vmas)
    {
    	return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,
    				       NULL, false,
    				       gup_flags | FOLL_TOUCH | FOLL_REMOTE);
    }
    EXPORT_SYMBOL(get_user_pages_remote);
    
    /*
     * This is the same as get_user_pages_remote(), just with a
     * less-flexible calling convention where we assume that the task
     * and mm being operated on are the current task's.  We also
     * obviously don't pass FOLL_REMOTE in here.
     */
    long get_user_pages(unsigned long start, unsigned long nr_pages,
    		unsigned int gup_flags, struct page **pages,
    		struct vm_area_struct **vmas)
    {
    	return __get_user_pages_locked(current, current->mm, start, nr_pages,
    				       pages, vmas, NULL, false,
    				       gup_flags | FOLL_TOUCH);
    }
    EXPORT_SYMBOL(get_user_pages);
    
    /**
     * populate_vma_page_range() -  populate a range of pages in the vma.
     * @vma:   target vma
     * @start: start address
     * @end:   end address
     * @nonblocking:
     *
     * This takes care of mlocking the pages too if VM_LOCKED is set.
     *
     * return 0 on success, negative error code on error.
     *
     * vma->vm_mm->mmap_sem must be held.
     *
     * If @nonblocking is NULL, it may be held for read or write and will
     * be unperturbed.
     *