[PATCH] mm: tracking shared dirty pages

Tracking of dirty pages in shared writeable mmap()s.

The idea is simple: write protect clean shared writeable pages, catch the
write-fault, make writeable and set dirty.  On page write-back clean all the
PTE dirty bits and write protect them once again.

The implementation is a tad harder, mainly because the default
backing_dev_info capabilities were too loosely maintained.  Hence it is not
enough to test the backing_dev_info for cap_account_dirty.

The current heuristic is as follows, a VMA is eligible when:
 - its shared writeable
    (vm_flags & (VM_WRITE|VM_SHARED)) == (VM_WRITE|VM_SHARED)
 - it is not a 'special' mapping
    (vm_flags & (VM_PFNMAP|VM_INSERTPAGE)) == 0
 - the backing_dev_info is cap_account_dirty
    mapping_cap_account_dirty(vma->vm_file->f_mapping)
 - f_op->mmap() didn't change the default page protection

Page from remap_pfn_range() are explicitly excluded because their COW
semantics are already horrid enough (see vm_normal_page() in do_wp_page()) and
because they don't have a backing store anyway.

mprotect() is taught about the new behaviour as well.  However it overrides
the last condition.

Cleaning the pages on write-back is done with page_mkclean() a new rmap call.
It can be called on any page, but is currently only implemented for mapped
pages, if the page is found the be of a VMA that accounts dirty pages it will
also wrprotect the PTE.

Finally, in fs/buffers.c:try_to_free_buffers(); remove clear_page_dirty() from
under ->private_lock.  This seems to be safe, since ->private_lock is used to
serialize access to the buffers, not the page itself.  This is needed because
clear_page_dirty() will call into page_mkclean() and would thereby violate
locking order.

[dhowells@redhat.com: Provide a page_mkclean() implementation for NOMMU]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 4 insertions, 6 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index d799d896d74..8507ee9cd57 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1105,12 +1105,6 @@ 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
@@ -1128,6 +1122,10 @@ munmap_back:
         pgoff = vma->vm_pgoff;
         vm_flags = vma->vm_flags;
 
+        if (vma_wants_writenotify(vma))
+                vma->vm_page_prot =
+                        protection_map[vm_flags & (VM_READ|VM_WRITE|VM_EXEC)];
+
         if (!file || !vma_merge(mm, prev, addr, vma->vm_end,
                         vma->vm_flags, NULL, file, pgoff, vma_policy(vma))) {
                 file = vma->vm_file;