Re: [PATCH 1/2]: Fix BUG in cancel_dirty_pages on XFS

[David Chinner]

On Wed, Jan 24, 2007 at 01:13:55PM +0100, Peter Zijlstra wrote:
> On Wed, 2007-01-24 at 09:37 +1100, David Chinner wrote:
> > With the recent changes to cancel_dirty_pages(), XFS will
> > dump warnings in the syslog because it can truncate_inode_pages()
> > on dirty mapped pages.
> > 
> > I've determined that this is indeed correct behaviour for XFS
> > as this can happen in the case of races on mmap()d files with
> > direct I/O. In this case when we do a direct I/O read, we
> > flush the dirty pages to disk, then truncate them out of the
> > page cache. Unfortunately, between the flush and the truncate
> > the mmap could dirty the page again. At this point we toss a
> > dirty page that is mapped.
> 
> This sounds iffy, why not just leave the page in the pagecache if its
> mapped anyway?

Because then fsx fails.

> > None of the existing functions for truncating pages or invalidating
> > pages work in this situation. Invalidating a page only works for
> > non-dirty pages with non-dirty buffers, and they only work for
> > whole pages and XFS requires partial page truncation.
> > 
> > On top of that the page invalidation functions don't actually
> > call into the filesystem to invalidate the page and so the filesystem
> > can't actually invalidate the page properly (e.g. do stuff based on
> > private buffer head flags).
> 
> Have you seen the new launder_page() a_op? called from
> invalidate_inode_pages2_range()

No, but we can't use invalidate_inode_pages2_range() because it
doesn't handle partial pages. I tried that first and it left warnings
in the syslog and fsx failed.

> > So that leaves us needing to use truncate semantics and the problem
> > is that none of them unmap pages in a non-racy manner - if they
> > unmap pages they do it separately to the truncate of the page,
> > leading to races with mmap redirtying the page between the unmap and
> > the truncate ofthe page.
> 
> Isn't there still a race where the page fault path doesn't yet lock the
> page and can just reinsert it?

Yes, but it's a tiny race compared to the other mechanisms
available.

> Nick's pagefault rework should rid us of this by always locking the page
> in the fault path.

Yes, and that's what I'm relying on to fix the problem completely.
invalidate_inode_pages2_range() needs this fix as well to be race
free, so it's not like I'm introducing a new problem....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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