From: Andrew Morton <akpm@osdl.org>
To: suparna@in.ibm.com
Cc: daniel@osdl.org, janetmor@us.ibm.com, pbadari@us.ibm.com,
linux-aio@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH linux-2.6.1-rc1-mm1] filemap_fdatawait.patch
Date: Thu, 1 Jan 2004 20:36:12 -0800 [thread overview]
Message-ID: <20040101203612.15debff2.akpm@osdl.org> (raw)
In-Reply-To: <20040102042036.GA3311@in.ibm.com>
Suparna Bhattacharya <suparna@in.ibm.com> wrote:
>
> On Wed, Dec 31, 2003 at 03:42:39PM -0800, Andrew Morton wrote:
> > Daniel McNeil <daniel@osdl.org> wrote:
> > >
> > > The other potential race in filemap_fdatawait() was that it
> > > removed the page from the locked list while waiting for a writeback
> > > and if there was a 2nd filemap_fdatawait() running on another cpu,
> > > it would not wait for the page being written since it would never see
> > > it on the list.
> >
> > That would only happen if one thread or the other was not running under
> > i_sem. The only path I see doing that is in generic_file_direct_IO()?
>
> Yes, and we should simply fix generic_file_direct_IO to avoid doing so.
> We anyway issue filemap_fdatawait later with i_sem held.
>
> The race that we need to worry about is between background writeouts
> (which don't take i_sem) and filemap_fdatawrite/filemap_fdatawait - i.e
> the first one discussed.
Well Daniel has raised a second race here, betwen filemap_fdatawait() and
filemap_fdatwait(). The background writeback code does not execute
filemap_datawait() anyway, so no prob.
Yes, extending i_sem coverage in the case O_DIRECT reads should suit. In
the (vastly) common case mapping->nrpages is zero anyway, so we shouldn't
even enter that code.
> > > + /*
> > > + * If the page is locked, it might be in process of being
> > > + * setup for writeback but without PG_writeback set
> > > + * and with PG_dirty cleared.
> > > + * (PG_dirty is cleared BEFORE PG_writeback is set)
> > > + * So, wait for the PG_locked to clear, then start over.
> > > + */
> > > + if (PageLocked(page)) {
> > > + page_cache_get(page);
> > > + spin_unlock(&mapping->page_lock);
> > > + wait_on_page_locked(page);
> > > + page_cache_release(page);
> > > + goto restart;
> > > + }
> >
> > Why is this a problem which needs addressing here? If some other thread is
> > in the process of starting I/O against this page then the page must have
> > been clean when this thread ran filemap_fdatawrite()?
>
> This is the same race that we have been discussing (background writer
> pulled this page off io_pages, put it on locked pages but hasn't set
> PG_writeback as yet). To me it seemed that Daniel's solution was just an
> alternative to what you proposed - i.e. adding lock_page() to filemap_fdatawait.
> I have to think a little about the fix -- AFAICS but we are all talking
> about the same (real) problem here.
Yup. Realish, anyway. Unless we can demonstrate that this is the cause of
the O_DIRECT data-exposure problems, this race isn't really very
interesting. It should be plugged though I guess.
next prev parent reply other threads:[~2004-01-02 4:35 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <3FCD4B66.8090905@us.ibm.com>
2003-12-06 1:29 ` [PATCH linux-2.6.0-test10-mm1] dio-read-race-fix Daniel McNeil
2003-12-08 18:23 ` Daniel McNeil
2003-12-12 0:51 ` Daniel McNeil
2003-12-17 1:25 ` [PATCH linux-2.6.0-test10-mm1] filemap_fdatawait.patch Daniel McNeil
2003-12-17 2:03 ` Andrew Morton
2003-12-17 19:25 ` Daniel McNeil
2003-12-17 20:17 ` Janet Morgan
2003-12-31 9:18 ` Suparna Bhattacharya
2003-12-31 9:35 ` Andrew Morton
2003-12-31 9:55 ` Suparna Bhattacharya
2003-12-31 9:59 ` Andrew Morton
2003-12-31 10:09 ` Suparna Bhattacharya
2003-12-31 10:10 ` Andrew Morton
2003-12-31 10:48 ` Suparna Bhattacharya
2003-12-31 10:53 ` Andrew Morton
2003-12-31 10:54 ` Andrew Morton
2003-12-31 11:17 ` Andrew Morton
2003-12-31 22:34 ` [PATCH linux-2.6.1-rc1-mm1] filemap_fdatawait.patch Daniel McNeil
2003-12-31 22:41 ` [PATCH linux-2.6.1-rc1-mm1] aiodio_fallback_bio_count.patch Daniel McNeil
2003-12-31 23:46 ` Andrew Morton
2004-01-02 5:14 ` Suparna Bhattacharya
2004-01-02 7:46 ` Andrew Morton
2004-01-05 3:55 ` Suparna Bhattacharya
2004-01-05 5:06 ` Andrew Morton
2004-01-05 5:28 ` Suparna Bhattacharya
2004-01-05 5:28 ` Andrew Morton
2004-01-05 6:06 ` Suparna Bhattacharya
2004-01-05 6:14 ` Lincoln Dale
2003-12-31 22:47 ` [PATCH linux-2.6.1-rc1-mm1] dio_isize.patch Daniel McNeil
2003-12-31 23:42 ` [PATCH linux-2.6.1-rc1-mm1] filemap_fdatawait.patch Andrew Morton
2004-01-02 4:20 ` Suparna Bhattacharya
2004-01-02 4:36 ` Andrew Morton [this message]
2004-01-02 5:50 ` [PATCH linux-2.6.0-test10-mm1] filemap_fdatawait.patch Suparna Bhattacharya
2004-01-02 7:31 ` Andrew Morton
2004-01-05 13:49 ` Marcelo Tosatti
2004-01-05 20:27 ` Andrew Morton
2004-03-29 15:44 ` Marcelo Tosatti
2004-01-11 23:14 ` Janet Morgan
2004-01-11 23:44 ` Andrew Morton
2004-01-12 18:00 ` filemap_fdatawait.patch Daniel McNeil
2004-01-12 19:39 ` [PATCH linux-2.6.0-test10-mm1] filemap_fdatawait.patch Janet Morgan
2004-01-12 19:46 ` Daniel McNeil
2004-01-13 4:12 ` Janet Morgan
2003-12-30 4:53 ` [PATCH linux-2.6.0-test10-mm1] dio-read-race-fix Suparna Bhattacharya
2003-12-31 0:29 ` Daniel McNeil
2003-12-31 6:09 ` Suparna Bhattacharya
2004-01-08 23:55 ` Daniel McNeil
2004-01-09 3:55 ` Suparna Bhattacharya
2004-02-05 1:39 ` [PATCH 2.6.2-rc3-mm1] DIO read race fix Daniel McNeil
2004-02-05 1:54 ` Badari Pulavarty
2004-02-05 2:07 ` Andrew Morton
2004-02-05 2:54 ` Janet Morgan
2004-02-05 3:19 ` Andrew Morton
2004-02-05 3:43 ` Suparna Bhattacharya
2004-02-05 5:33 ` Andrew Morton
2004-02-05 17:52 ` Daniel McNeil
2004-02-05 18:53 ` Badari Pulavarty
2004-03-29 15:41 ` [PATCH linux-2.6.0-test10-mm1] dio-read-race-fix Suparna Bhattacharya
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