Date: Thu, 23 Jun 2011 08:18:04 +0200
From: Andrea Righi <andrea@betterlinux.com>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: akpm@linux-foundation.org, minchan.kim@gmail.com, riel@redhat.com,
        peterz@infradead.org, hannes@cmpxchg.org,
        kamezawa.hiroyu@jp.fujitsu.com, aarcange@redhat.com, hughd@google.com,
        jamesjer@betterlinux.com, marcus@bluehost.com, matt@bluehost.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC] fadvise: move active pages to inactive list with
 POSIX_FADV_DONTNEED
Message-ID: <20110623061803.GA1311@thinkpad>
References: <1308779480-4950-1-git-send-email-andrea@betterlinux.com>
 <4E02AFCE.6000604@jp.fujitsu.com>
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On Thu, Jun 23, 2011 at 12:15:26PM +0900, KOSAKI Motohiro wrote:
> (2011/06/23 6:51), Andrea Righi wrote:
> > There were some reported problems in the past about trashing page cache
> > when a backup software (i.e., rsync) touches a huge amount of pages (see
> > for example [1]).
> > 
> > This problem has been almost fixed by the Minchan Kim's patch [2] and a
> > proper use of fadvise() in the backup software. For example this patch
> > set [3] has been proposed for inclusion in rsync.
> > 
> > However, there can be still other similar trashing problems: when the
> > backup software reads all the source files, some of them may be part of
> > the actual working set of the system. When a
> > posix_fadvise(POSIX_FADV_DONTNEED) is performed _all_ pages are evicted
> > from pagecache, both the working set and the use-once pages touched only
> > by the backup software.
> > 
> > With the following solution when posix_fadvise(POSIX_FADV_DONTNEED) is
> > called for an active page instead of removing it from the page cache it
> > is added to the tail of the inactive list. Otherwise, if it's already in
> > the inactive list the page is removed from the page cache.
> > 
> > In this way if the backup was the only user of a page, that page will
> > be immediately removed from the page cache by calling
> > posix_fadvise(POSIX_FADV_DONTNEED). If the page was also touched by
> > other processes it'll be moved to the inactive list, having another
> > chance of being re-added to the working set, or simply reclaimed when
> > memory is needed.
> > 
> > Testcase:
> > 
> >   - create a 1GB file called "zero"
> >   - run md5sum zero to read all the pages in page cache (this is to
> >     simulate the user activity on this file)
> >   - run "rsync zero zero_copy" (rsync is patched with [3])
> >   - re-run md5sum zero (user activity on the working set) and measure
> >     the time to complete this command
> > 
> > The test has been performed using 3.0.0-rc4 vanilla and with this patch
> > applied (3.0.0-rc4-fadvise).
> > 
> > Results:
> >                   avg elapsed time      block:block_bio_queue
> >  3.0.0-rc4                  4.127s                      8,214
> >  3.0.0-rc4-fadvise          2.146s                          0
> > 
> > In the first case the file is evicted from page cache completely and we
> > must re-read it from the disk. In the second case the file is still in
> > page cache (in the inactive list) and we don't need any other additional
> > I/O operation.
> > 
> > [1] http://marc.info/?l=rsync&m=128885034930933&w=2
> > [2] https://lkml.org/lkml/2011/2/20/57
> > [3] http://lists.samba.org/archive/rsync/2010-November/025827.html
> > 
> > Signed-off-by: Andrea Righi <andrea@betterlinux.com>
> > ---
> >  mm/swap.c     |    9 +++++----
> >  mm/truncate.c |    5 ++++-
> >  2 files changed, 9 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/swap.c b/mm/swap.c
> > index 3a442f1..fc8bb76 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -411,10 +411,11 @@ void add_page_to_unevictable_list(struct page *page)
> >   *
> >   * 1. active, mapped page -> none
> >   * 2. active, dirty/writeback page -> inactive, head, PG_reclaim
> > - * 3. inactive, mapped page -> none
> > - * 4. inactive, dirty/writeback page -> inactive, head, PG_reclaim
> > - * 5. inactive, clean -> inactive, tail
> > - * 6. Others -> none
> > + * 3. active, clean -> inactive, tail
> > + * 4. inactive, mapped page -> none
> > + * 5. inactive, dirty/writeback page -> inactive, head, PG_reclaim
> > + * 6. inactive, clean -> inactive, tail
> > + * 7. Others -> none
> >   *
> >   * In 4, why it moves inactive's head, the VM expects the page would
> >   * be write it out by flusher threads as this is much more effective
> > diff --git a/mm/truncate.c b/mm/truncate.c
> > index 3a29a61..043aabd 100644
> > --- a/mm/truncate.c
> > +++ b/mm/truncate.c
> > @@ -357,7 +357,10 @@ unsigned long invalidate_mapping_pages(struct address_space *mapping,
> >  			if (lock_failed)
> >  				continue;
> >  
> > -			ret = invalidate_inode_page(page);
> > +			if (PageActive(page))
> > +				ret = 0;
> > +			else
> > +				ret = invalidate_inode_page(page);
> 
> So, after this patch, following comment is a bit outdated. we deactivate
> the page even if it's not invalidated.
> 
>                         /*
>                          * Invalidation is a hint that the page is no longer
>                          * of interest and try to speed up its reclaim.
>                          */
> 
> Can you please fix the comment too? Other than that,
> 	Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

OK, will fix the comment and post a new version soon.

Thanks for reviewing,
-Andrea
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