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

[Darrick J. Wong]

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).

Unfortunately, ext4_page_mkwrite will check for page locked, wait for page
writeback, and then return the page.  I think it is theoretically possible for
#1 to trigger a page_mkwrite which completes before #2b, right?  In which case
the thread that called mkwrite will think that the page isn't being written
out, and happily scribble on it during writeback.  I could be wrong, but it
seems to me that one has to write-protect the page after setting the writeback
bit.

I guess we could call page_mkclean from bio_integrity_prep, though.

--D
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