[rfc][patch] mm: dirty page accounting hole

[Nick Piggin <nickpiggin@yahoo.com.au>]

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Hi,

I think I'm running into a hole in dirty page accounting...

What seems to be happening is that a page gets written to via a
VM_SHARED vma. We then set the pte dirty, then mark the page dirty.
Next, mprotect changes the vma so it is no longer writeable so it
is no longer VM_SHARED. The pte is still dirty.

Then clear_page_dirty_for_io is called and leaves that pte dirty
and cleans the page. It never gets cleaned until munmap, so msync
and writeout accounting are broken.

I have a fix which just scans VM_SHARED to VM_MAYSHARE. The other
way I tried is to clear the dirty and write bits and set the page
dirty in mprotect. The problem with that for me is that I'm trying
to rework the vm/fs layer so we never have to allocate data to
write out dirty pages (using page_mkwrite and dirty accounting),
and so this still leaves me with a window where the vma flags are
changed but before the pte is marked clean, in which time the page
is still dirty but it may have its metadata freed because it
doesn't look dirty.

There are several other problems I've also run into, including a
fundamentally indadequate page_mkwrite locking scheme, which was
naturally ignored when I brought it up during reviewing those
patches. I digress...

Anyway, here's a patch to fix this first particular issue...

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Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -481,7 +481,7 @@ static int page_mkclean_file(struct addr
 
 	spin_lock(&mapping->i_mmap_lock);
 	vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
-		if (vma->vm_flags & VM_SHARED)
+		if (vma->vm_flags & VM_MAYSHARE)
 			ret += page_mkclean_one(page, vma);
 	}
 	spin_unlock(&mapping->i_mmap_lock);