Re: [RFC v2] block integrity: Stabilize(?) pages during writeback

[Darrick J. Wong]

On Tue, Apr 26, 2011 at 07:33:18AM -0400, Chris Mason wrote:
> Excerpts from Darrick J. Wong's message of 2011-04-25 20:37:38 -0400:
> > On Fri, Apr 22, 2011 at 10:34:34PM +0200, Jan Kara wrote:
> > > On Fri 22-04-11 08:50:01, Chris Mason wrote:
> > > > Excerpts from Darrick J. Wong's message of 2011-04-21 20:02:26 -0400:
> > > > > Hi everyone,
> > > > > 
> > > > > I've finally managed to get together a patch that seems to provide stable pages
> > > > > during writeback, or at least gets us to the point that after several days of
> > > > > running tests I don't see DIF checksum errors anymore. :)
> > > > > 
> > > > > The last two pieces to go into this puzzle were (a) bio_integrity_prep needs to
> > > > > walk the process tree to find all userland ptes that map to a particular memory
> > > > > page and revoke write access, and
> > > > 
> > > > Hmm, did you need the bio_integrity_prep change for all the filesystems?
> > > > This should be happening already as part of using page_mkwrite.
> > >   Or more precisely page_mkclean() should do what you try to do in
> > > bio_integrity_prep()... It would certainly be interesting (bug) if you
> > > could write to the page after calling page_mkclean() without page_mkwrite()
> > > being called.
> > 
> > Hm... in mpage_da_submit_io I see the following sequence of calls:
> > 
> > 1. clear_page_dirty_for_io
> > 2. possibly one of: ext4_bio_write_page or block_write_full_page.
> >    If ext4_bio_write_page, 
> >    2a. kmem_cache_alloc
> >    2b. set_page_writeback
> > 
> > Before and after #1, the page is locked but writeback is not set.
> > 
> > Before #2, the page must be locked and writeback must not be set, because both
> > of those two functions want to set the writeback bit themselves.  However,
> > ext4_bio_write_page tries to allocate memory with GFP_NOFS, which means it can
> > sleep (I think).
> 
> Sleeping isn't the problem as long as you sleep with the page locked.
> The idea is that writepage will:
> 
> 1) lock the page
> 2) clear_page_dirty_for_io (which calls page_mkclean)
> 3) set_page_writeback()
> 4) unlock the page
> 5) start the IO
> 
> page_mkwrite will:
> 
> 1) lock the page
> 2) wait on page writeback
> 3) do other stuff
> 
> So if ext is calling set_page_writeback() on an unlocked page, that's a
> problem.  Otherwise it should be working.

You're right, at this point in time writepage and page_mkwrite in ext4 both
behave as you describe.  I began backing out parts of my patches to
bio-integrity.c and discovered that with the current kernel (2.6.39-rc5) the
only part that seems useful is the set_memory_ro/rw pair from that old
debugging patch.  Unfortunately, those two functions only seem to exist on x86;
I suppose I could port them to others.  If that's even a sane idea.

--D
> 
> -chris
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