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Subject[PATCH 08/14] FS-Cache: Add notification of page becoming writable to VMA ops [try #10]
Date
The attached patch adds 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.

Signed-Off-By: David Howells <dhowells@redhat.com>
---
include/linux/mm.h | 4 ++
mm/memory.c | 99 +++++++++++++++++++++++++++++++++++++++-------------
mm/mmap.c | 12 +++++-
mm/mprotect.c | 11 +++++-
4 files changed, 98 insertions(+), 28 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1154684..cd3c2cf 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -200,6 +200,10 @@ 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,
diff --git a/mm/memory.c b/mm/memory.c
index 0ec7bc6..6c6891e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1445,25 +1445,59 @@ static int do_wp_page(struct mm_struct *
{
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);
- unlock_page(old_page);
- if (reuse) {
- flush_cache_page(vma, address, pte_pfn(orig_pte));
- entry = pte_mkyoung(orig_pte);
- entry = maybe_mkwrite(pte_mkdirty(entry), vma);
- ptep_set_access_flags(vma, address, page_table, entry, 1);
- update_mmu_cache(vma, address, entry);
- lazy_mmu_prot_update(entry);
- ret |= VM_FAULT_WRITE;
- goto unlock;
+ if (unlikely(vma->vm_flags & VM_SHARED)) {
+ 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);
+ entry = maybe_mkwrite(pte_mkdirty(entry), vma);
+ ptep_set_access_flags(vma, address, page_table, entry, 1);
+ update_mmu_cache(vma, address, entry);
+ lazy_mmu_prot_update(entry);
+ ret |= VM_FAULT_WRITE;
+ goto unlock;
}

/*
@@ -1523,6 +1557,10 @@ oom:
if (old_page)
page_cache_release(old_page);
return VM_FAULT_OOM;
+
+unwritable_page:
+ page_cache_release(old_page);
+ return VM_FAULT_SIGBUS;
}

/*
@@ -2074,18 +2112,31 @@ retry:
/*
* Should we do an early C-O-W break?
*/
- if (write_access && !(vma->vm_flags & VM_SHARED)) {
- struct page *page;
+ if (write_access) {
+ if (!(vma->vm_flags & VM_SHARED)) {
+ struct page *page;

- if (unlikely(anon_vma_prepare(vma)))
- goto oom;
- page = alloc_page_vma(GFP_HIGHUSER, vma, address);
- if (!page)
- goto oom;
- copy_user_highpage(page, new_page, address);
- page_cache_release(new_page);
- new_page = page;
- anon = 1;
+ if (unlikely(anon_vma_prepare(vma)))
+ goto oom;
+ page = alloc_page_vma(GFP_HIGHUSER, vma, address);
+ if (!page)
+ goto oom;
+ copy_user_highpage(page, new_page, address);
+ 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);
diff --git a/mm/mmap.c b/mm/mmap.c
index e6ee123..6446c61 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -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,
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;
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 4c14d42..2697abd 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -106,6 +106,7 @@ mprotect_fixup(struct vm_area_struct *vm
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;
@@ -132,8 +133,6 @@ mprotect_fixup(struct vm_area_struct *vm
}
}

- newprot = protection_map[newflags & 0xf];
-
/*
* First try to merge with previous and/or next vma.
*/
@@ -160,6 +159,14 @@ mprotect_fixup(struct vm_area_struct *vm
}

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.
-
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