Re: [PATCH v2] fadvise: move active pages to inactive list with POSIX_FADV_DONTNEED

[Minchan Kim]

Hi Andrea,
Sorry for late response.
These day, I have no time to see the LKML.

On Thu, Jun 23, 2011 at 3:36 PM, Andrea Righi <andrea@xxxxxxxxxxxxxxx> 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.

Agreed. It's rather aggressive.

>
> 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:
>
> Â- create a 1GB file called "zero"
> Â- run md5sum zero to read all the pages in page cache (this is to
> Â Âsimulate the user activity on this file)
> Â- run "rsync zero zero_copy" (rsync is patched with [3])
> Â- re-run md5sum zero (user activity on the working set) and measure
> Â Âthe 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:
>         Âavg elapsed time   Âblock:block_bio_queue
> Â3.0.0-rc4         Â4.127s           Â8,214
> Â3.0.0-rc4-fadvise     Â2.146s             Â0
>

Great!

> 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:
> Â- fix comment in invalidate_mapping_pages()
>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> Signed-off-by: Andrea Righi <andrea@xxxxxxxxxxxxxxx>
> ---
> Âmm/swap.c   |  Â9 +++++----
> Âmm/truncate.c | Â 10 +++++++---
> Â2 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)
> Â*
> Â* 1. active, mapped page -> none
> Â* 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

Nitpick.
I would like to put together them by on line as rather than adding another line.
5, [in]active, clean-> inactive, tail.
I guess it's more easy to understand.

If you want to put it in another line, please change below comment, too.
"In 5, why it moves inactive's head.."

> Â*
> Â* In 4, why it moves inactive's head, the VM expects the page would
> Â* 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,
> Â Â Â Â Â Â Â Â Â Â Â Âif (lock_failed)
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Âcontinue;
>
> - Â Â Â Â Â Â Â Â Â Â Â ret = invalidate_inode_page(page);

I would like to add comment.
"Invalidation of active page is rather aggressive as we can't make
sure it's not a working set of other processes.
deactivate_page would move it into inactive's tail so the page will
have a chance to activate again if other processes
touch it. otherwise, it would be reclaimed simply".

> + Â Â Â Â Â Â Â Â Â Â Â if (PageActive(page))
> + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â ret = 0;
> + Â Â Â Â Â Â Â Â Â Â Â else
> + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â ret = invalidate_inode_page(page);


You have to change description of invalidate_mapping_pages.

 * invalidate_mapping_pages() will not block on IO activity. It will not
 * invalidate pages which are dirty, locked, under writeback, mapped into
 * pagetables or on active lru.

> Â Â Â Â Â Â Â Â Â Â Â Âunlock_page(page);
> Â Â Â Â Â Â Â Â Â Â Â Â/*
> - Â Â Â Â Â Â Â Â Â Â Â Â* Invalidation is a hint that the page is no longer
> - Â Â Â Â Â Â Â Â Â Â Â Â* of interest and try to speed up its reclaim.
> + Â Â Â Â Â Â Â Â Â Â Â Â* Invalidation of an inactive page is a hint that the
> + Â Â Â Â Â Â Â Â Â Â Â Â* page is no longer of interest and try to speed up
> + Â Â Â Â Â Â Â Â Â Â Â Â* its reclaim.
> Â Â Â Â Â Â Â Â Â Â Â Â */
> Â Â Â Â Â Â Â Â Â Â Â Âif (!ret)
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Âdeactivate_page(page);
> --
> 1.7.4.1
>
>

Otherwise, Looks good to me.

Acked-by: Minchan Kim <minchan.kim@xxxxxxxxx>

-- 
Kind regards,
Minchan Kim
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