Date:	Fri, 24 Jun 2011 13:57:54 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Andrea Righi <andrea@...terlinux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Jerry James <jamesjer@...terlinux.com>,
	Marcus Sorensen <marcus@...ehost.com>,
	Matt Heaton <matt@...ehost.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] fadvise: move active pages to inactive list with
 POSIX_FADV_DONTNEED

On Thu, 2011-06-23 at 14:36 +0800, 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
> 
> ChangeLog v1 -> v2:
>  - fix comment in invalidate_mapping_pages()
> 
> Acked-by: Rik van Riel <riel@...hat.com>
> Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Signed-off-by: Andrea Righi <andrea@...terlinux.com>
> ---
>  mm/swap.c     |    9 +++++----
>  mm/truncate.c |   10 +++++++---
>  2 files changed, 12 insertions(+), 7 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..a36af48 100644
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -357,11 +357,15 @@ 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);
>  			unlock_page(page);
>  			/*
> -			 * Invalidation is a hint that the page is no longer
> -			 * of interest and try to speed up its reclaim.
> +			 * Invalidation of an inactive page is a hint that the
> +			 * page is no longer of interest and try to speed up
> +			 * its reclaim.
>  			 */
>  			if (!ret)
>  				deactivate_page(page);
this looks changed behavior, active page will not be invalidated.
invalidate_mapping_pages is not just used by fadvise. for
example, /proc/sys/vm/drop_cache can't drop active pages any more with
the patch in the first invoke. Please audit other use cases too.

Thanks,
Shaohua


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