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

Hugh Dickins
2008-08-12T11:15:58Z

On Tue, 12 Aug 2008, Nick Piggin wrote:
>
> 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.

I don't think you've got that right yet.

mprotect can of course change vma->vm_flags to take VM_WRITE off,
making vma no longer writeable; but it shouldn't be touching
VM_SHARED. And a quick check with debugger confirms that.

It's precisely because of mprotect that page_mkclean_one tests
VM_SHARED not VM_WRITE. Changing that to VM_MAYSHARE, as in your
patch below, should make no difference to correctness; but would
potentially make its loop less efficient (it would also go off to
check MAP_SHARED, PROT_READ, fd readonly mappings unnecessarily).

Perhaps there's somewhere else that clears VM_SHARED by mistake?
Or another path through mprotect which does so? I haven't checked
further, hoping this will jolt you into a different realization.

>
> 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.

While I disagree with the patch itself, and don't understand the
details of what you're working on there, I certainly agree that it's
better for mprotect not to set the pages dirty: at present (on some
arches, in most cases? it changes from time to time) change_pte_range
is an operation on ptes which doesn't have to mess with struct pages.

>
> 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...

Unsatisfactory, yes, sorry about that;
but no, you're not "naturally ignored".

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

Could you please go back to inlining your patches?

Thanks,
Hugh

>
> 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);