linux-kernel - Re: [GIT PULL] afs: Fixes

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Message-ID: <20171125224806.GM4094@dastard>
Date:   Sun, 26 Nov 2017 09:48:06 +1100
From:   Dave Chinner <david@...morbit.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-afs@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] afs: Fixes

On Sat, Nov 25, 2017 at 10:35:43PM +0000, David Howells wrote:
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > So I see in the commit message why afs needs to do this, but it's
> > worth pointing out that it's
> > 
> >  (a) impossible to avoid the "inconsistent data" case for writable mmap'ed
> >  pages
> 
> Doesn't clear_page_dirty_for_io() write-protect the PTE for the page to be
> written out, in which case ->page_mkwrite() will get called again before the
> page is redirtied?

Yes, but page_mkwrite will only block on writeback in progress is if
the backing device says it needs stable pages.  See
wait_for_stable_page().  e.g. stable pages are required if RAID is
in use, otherwise modification during IO can result in broken
on-disk parity/mirroring....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com