[Ocfs2-devel] [PATCH] Add notification of page becoming writable to VMA ops
David Howells
dhowells at redhat.com
Tue Jun 20 08:20:21 CDT 2006
From: David Howells <dhowells at redhat.com>
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.
The patch has been fixed to only check pages in shared writable VMAs in
do_wp_page(), not in all shared VMAs.
Signed-Off-By: David Howells <dhowells at redhat.com>
---
include/linux/mm.h | 4 ++
mm/memory.c | 100 ++++++++++++++++++++++++++++++++++++++++------------
mm/mmap.c | 12 +++++-
mm/mprotect.c | 11 +++++-
4 files changed, 99 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..b7d7e29 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1445,25 +1445,60 @@ 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|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);
+ 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 +1558,10 @@ 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 +2113,31 @@ 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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