Commit 9637a5ef authored by David Howells's avatar David Howells Committed by Linus Torvalds
Browse files

[PATCH] add page_mkwrite() vm_operations method 



Add a new VMA operation to notify a filesystem or other driver about the
MMU generating a fault because userspace attempted to write to a page
mapped through a read-only PTE.

This facility permits the filesystem or driver to:

 (*) Implement storage allocation/reservation on attempted write, and so to
     deal with problems such as ENOSPC more gracefully (perhaps by generating
     SIGBUS).

 (*) Delay making the page writable until the contents have been written to a
     backing cache. This is useful for NFS/AFS when using FS-Cache/CacheFS.
     It permits the filesystem to have some guarantee about the state of the
     cache.

 (*) Account and limit number of dirty pages. This is one piece of the puzzle
     needed to make shared writable mapping work safely in FUSE.

Needed by cachefs (Or is it cachefiles?  Or fscache? <head spins>).

At least four other groups have stated an interest in it or a desire to use
the functionality it provides: FUSE, OCFS2, NTFS and JFFS2.  Also, things like
EXT3 really ought to use it to deal with the case of shared-writable mmap
encountering ENOSPC before we permit the page to be dirtied.

From: Peter Zijlstra <a.p.zijlstra@chello.nl>

  get_user_pages(.write=1, .force=1) can generate COW hits on read-only
  shared mappings, this patch traps those as mkpage_write candidates and fails
  to handle them the old way.

Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Joel Becker <Joel.Becker@oracle.com>
Cc: Mark Fasheh <mark.fasheh@oracle.com>
Cc: Anton Altaparmakov <aia21@cantab.net>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent bd96b9eb

include/linux/mm.h

+4 −0
Original line number Diff line number Diff line
@@ -199,6 +199,10 @@ struct vm_operations_struct { 
	void (*close)(struct vm_area_struct * area);

	struct page * (*nopage)(struct vm_area_struct * area, unsigned long address, int *type);

	int (*populate)(struct vm_area_struct * area, unsigned long address, unsigned long len, pgprot_t prot, unsigned long pgoff, int nonblock);



	/* notification that a previously read-only page is about to become

	 * writable, if an error is returned it will cause a SIGBUS */

	int (*page_mkwrite)(struct vm_area_struct *vma, struct page *page);

#ifdef CONFIG_NUMA

	int (*set_policy)(struct vm_area_struct *vma, struct mempolicy *new);

	struct mempolicy *(*get_policy)(struct vm_area_struct *vma,

mm/memory.c

+76 −24
Original line number Diff line number Diff line
@@ -1457,15 +1457,51 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma, 
{

	struct page *old_page, *new_page;

	pte_t entry;

	int ret = VM_FAULT_MINOR;

	int reuse, ret = VM_FAULT_MINOR;



	old_page = vm_normal_page(vma, address, orig_pte);

	if (!old_page)

		goto gotten;



	if (PageAnon(old_page) && !TestSetPageLocked(old_page)) {

		int reuse = can_share_swap_page(old_page);

	if (unlikely((vma->vm_flags & (VM_SHARED|VM_WRITE)) ==

				(VM_SHARED|VM_WRITE))) {

		if (vma->vm_ops && vma->vm_ops->page_mkwrite) {

			/*

			 * Notify the address space that the page is about to

			 * become writable so that it can prohibit this or wait

			 * for the page to get into an appropriate state.

			 *

			 * We do this without the lock held, so that it can

			 * sleep if it needs to.

			 */

			page_cache_get(old_page);

			pte_unmap_unlock(page_table, ptl);



			if (vma->vm_ops->page_mkwrite(vma, old_page) < 0)

				goto unwritable_page;



			page_cache_release(old_page);



			/*

			 * Since we dropped the lock we need to revalidate

			 * the PTE as someone else may have changed it.  If

			 * they did, we just return, as we can count on the

			 * MMU to tell us if they didn't also make it writable.

			 */

			page_table = pte_offset_map_lock(mm, pmd, address,

							 &ptl);

			if (!pte_same(*page_table, orig_pte))

				goto unlock;

		}



		reuse = 1;

	} else if (PageAnon(old_page) && !TestSetPageLocked(old_page)) {

		reuse = can_share_swap_page(old_page);

		unlock_page(old_page);

	} else {

		reuse = 0;

	}



	if (reuse) {

		flush_cache_page(vma, address, pte_pfn(orig_pte));

		entry = pte_mkyoung(orig_pte);
@@ -1476,7 +1512,6 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma, 
		ret |= VM_FAULT_WRITE;

		goto unlock;

	}

	}



	/*

	 * Ok, we need to copy. Oh, well..
@@ -1535,6 +1570,10 @@ oom: 
	if (old_page)

		page_cache_release(old_page);

	return VM_FAULT_OOM;



unwritable_page:

	page_cache_release(old_page);

	return VM_FAULT_SIGBUS;

}



/*
@@ -2083,7 +2122,8 @@ retry: 
	/*

	 * Should we do an early C-O-W break?

	 */

	if (write_access && !(vma->vm_flags & VM_SHARED)) {

	if (write_access) {

		if (!(vma->vm_flags & VM_SHARED)) {

			struct page *page;



			if (unlikely(anon_vma_prepare(vma)))
@@ -2095,6 +2135,18 @@ retry: 
			page_cache_release(new_page);

			new_page = page;

			anon = 1;



		} else {

			/* if the page will be shareable, see if the backing

			 * address space wants to know that the page is about

			 * to become writable */

			if (vma->vm_ops->page_mkwrite &&

			    vma->vm_ops->page_mkwrite(vma, new_page) < 0

			    ) {

				page_cache_release(new_page);

				return VM_FAULT_SIGBUS;

			}

		}

	}



	page_table = pte_offset_map_lock(mm, pmd, address, &ptl);

mm/mmap.c

+10 −2
Original line number Diff line number Diff line
@@ -1065,7 +1065,8 @@ munmap_back: 
	vma->vm_start = addr;

	vma->vm_end = addr + len;

	vma->vm_flags = vm_flags;

	vma->vm_page_prot = protection_map[vm_flags & 0x0f];

	vma->vm_page_prot = protection_map[vm_flags &

				(VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)];

	vma->vm_pgoff = pgoff;



	if (file) {
@@ -1089,6 +1090,12 @@ munmap_back: 
			goto free_vma;

	}



	/* Don't make the VMA automatically writable if it's shared, but the

	 * backer wishes to know when pages are first written to */

	if (vma->vm_ops && vma->vm_ops->page_mkwrite)

		vma->vm_page_prot =

			protection_map[vm_flags & (VM_READ|VM_WRITE|VM_EXEC)];



	/* We set VM_ACCOUNT in a shared mapping's vm_flags, to inform

	 * shmem_zero_setup (perhaps called through /dev/zero's ->mmap)

	 * that memory reservation must be checked; but that reservation
@@ -1921,7 +1928,8 @@ unsigned long do_brk(unsigned long addr, unsigned long len) 
	vma->vm_end = addr + len;

	vma->vm_pgoff = pgoff;

	vma->vm_flags = flags;

	vma->vm_page_prot = protection_map[flags & 0x0f];

	vma->vm_page_prot = protection_map[flags &

				(VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)];

	vma_link(mm, vma, prev, rb_link, rb_parent);

out:

	mm->total_vm += len >> PAGE_SHIFT;

mm/mprotect.c

+9 −2
Original line number Diff line number Diff line
@@ -123,6 +123,7 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev, 
	unsigned long oldflags = vma->vm_flags;

	long nrpages = (end - start) >> PAGE_SHIFT;

	unsigned long charged = 0;

	unsigned int mask;

	pgprot_t newprot;

	pgoff_t pgoff;

	int error;
@@ -149,8 +150,6 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev, 
		}

	}



	newprot = protection_map[newflags & 0xf];



	/*

	 * First try to merge with previous and/or next vma.

	 */
@@ -177,6 +176,14 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev, 
	}



success:

	/* Don't make the VMA automatically writable if it's shared, but the

	 * backer wishes to know when pages are first written to */

	mask = VM_READ|VM_WRITE|VM_EXEC|VM_SHARED;

	if (vma->vm_ops && vma->vm_ops->page_mkwrite)

		mask &= ~VM_SHARED;



	newprot = protection_map[newflags & mask];



	/*

	 * vm_flags and vm_page_prot are protected by the mmap_sem

	 * held in write mode.

CRA Git | Maintained and supported by SUSTech CRA and CCSE