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

Peter Zijlstra
2008-08-12T06:50:40Z

On Tue, 2008-08-12 at 15:58 +1000, Nick Piggin wrote:
>
> 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),

Ooh, nice!

> 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...

Yes, I remember you bringing that up, and later too when you did those
fault patches. I assumed you were 'working' on it.

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

Looks good.
>
> 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);