Re: ftruncate-mmap: pages are lost after writing to mmaped file.

[Jan Kara <jack@suse.cz>]

  Hi Ying,

On Wed 01-04-09 15:36:13, Ying Han wrote:
>     I feel that the problem you saw is kind of differnt than mine. As
> you mentioned that you saw the PageError() message, which i don't see
> it on my system. I tried you patch(based on 2.6.21) on my system and
> it runs ok for 2 days, Still, since i don't see the same error message
> as you saw, i am not convineced this is the root cause at least for
> our problem. I am still looking into it.
>     So, are you seeing the PageError() every time the problem happened?
  Yes, but I agree that your problem is probably different. BTW: How do you
reproduce the problem?

								Honza

> On Tue, Mar 24, 2009 at 10:35 AM, Jan Kara <jack@suse.cz> wrote:
> > On Tue 24-03-09 16:48:14, Jan Kara wrote:
> >> On Wed 25-03-09 02:03:54, Nick Piggin wrote:
> >> > On Wednesday 25 March 2009 01:47:09 Jan Kara wrote:
> >> > > On Wed 25-03-09 01:30:00, Nick Piggin wrote:
> >> >
> >> > > > I don't think it is a very good idea for block_write_full_page recovery
> >> > > > to do clear_buffer_dirty for !mapped buffers. I think that should rather
> >> > > > be a redirty_page_for_writepage in the case that the buffer is dirty.
> >> > > >
> >> > > > Perhaps not the cleanest way to solve the problem if it is just due to
> >> > > > transient shortage of space in ext3, but generic code shouldn't be
> >> > > > allowed to throw away dirty data even if it can't be written back due
> >> > > > to some software or hardware error.
> >> > >
> >> > >   Well, that would be one possibility. But then we'd be left with dirty
> >> > > pages we cannot ever release since they are constantly dirty (when the
> >> > > filesystem really becomes out of space). So what I
> >> >
> >> > If the filesystem becomes out of space and we have over-committed these
> >> > dirty mmapped blocks, then we most definitely want to keep them around.
> >> > An error of the system losing a few pages (or if it happens an insanely
> >> > large number of times, then slowly dying due to memory leak) is better
> >> > than an app suddenly seeing the contents of the page change to nulls
> >> > under it when the kernel decides to do some page reclaim.
> >>   Hmm, probably you're right. Definitely it would be much easier to track
> >> the problem down than it is now... Thinking a bit more... But couldn't a
> >> malicious user bring the machine easily to OOM this way? That would be
> >> unfortunate.
> >  OK, below is the patch which makes things work for me (i.e. no data
> > lost). What do you think?
> >
> >                                                                        Honza
> > --
> > Jan Kara <jack@suse.cz>
> > SUSE Labs, CR
> >
> > From f423c2964dd5afbcc40c47731724d48675dd2822 Mon Sep 17 00:00:00 2001
> > From: Jan Kara <jack@suse.cz>
> > Date: Tue, 24 Mar 2009 16:38:22 +0100
> > Subject: [PATCH] fs: Don't clear dirty bits in block_write_full_page()
> >
> > If getblock() fails in block_write_full_page(), we don't want to clear
> > dirty bits on buffers. Actually, we even want to redirty the page. This
> > way we just won't silently discard users data (written e.g. through mmap)
> > in case of ENOSPC, EDQUOT, EIO or other write error. The downside of this
> > approach is that if the error is persistent we have this page pinned in
> > memory forever and if there are lots of such pages, we can bring the
> > machine OOM.
> >
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  fs/buffer.c |   10 +++-------
> >  1 files changed, 3 insertions(+), 7 deletions(-)
> >
> > diff --git a/fs/buffer.c b/fs/buffer.c
> > index 891e1c7..ae779a0 100644
> > --- a/fs/buffer.c
> > +++ b/fs/buffer.c
> > @@ -1833,9 +1833,11 @@ recover:
> >        /*
> >         * ENOSPC, or some other error.  We may already have added some
> >         * blocks to the file, so we need to write these out to avoid
> > -        * exposing stale data.
> > +        * exposing stale data. We redirty the page so that we don't
> > +        * loose data we are unable to write.
> >         * The page is currently locked and not marked for writeback
> >         */
> > +       redirty_page_for_writepage(wbc, page);
> >        bh = head;
> >        /* Recovery: lock and submit the mapped buffers */
> >        do {
> > @@ -1843,12 +1845,6 @@ recover:
> >                    !buffer_delay(bh)) {
> >                        lock_buffer(bh);
> >                        mark_buffer_async_write(bh);
> > -               } else {
> > -                       /*
> > -                        * The buffer may have been set dirty during
> > -                        * attachment to a dirty page.
> > -                        */
> > -                       clear_buffer_dirty(bh);
> >                }
> >        } while ((bh = bh->b_this_page) != head);
> >        SetPageError(page);
> > --
> > 1.6.0.2
> >
> > --
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> >
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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