Re: [PATCH 2.6.3-rc2-mm1] address_space_serialize_writeback patch

[Andrew Morton <akpm@osdl.org>]

Daniel McNeil <daniel@osdl.org> wrote:
>
> Here is the patch that does what you suggested.  It adds a rwsema to
> the address_space and do_writepages() uses it serialize writebacks.

OK, but we're only holding the rwsem across filemap_fdatawrite().  What
happens after we've dropped the rwsem and we are running
filemap_fdatawait()?  Cannot kupdate come in and start moving pages onto
the wrong address_space lists while filemap_fdatawait() is trying to wait
on them?

I think so.  Possibly your test just doesn't cover this case.

If so then we need to hold the rwsem for writing across the entire
write-and-wait.  And that is going to rather suck if and when we bring back
the sync_page_range() patch, which permitted concurrent fsync() against
different fd's which cover different parts of the file.

We need to check that we're bypassing all this stuff for access to
blockdevs too - we have no security issues to worry about there.