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

Minchan Kim
2011-06-23T22:06:45Z

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@betterlinux.com> 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:
>
>  - 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
>

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:
>  - fix comment in invalidate_mapping_pages()
>
> Acked-by: Rik van Riel <riel@redhat.com>
> Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Signed-off-by: Andrea Righi <andrea@betterlinux.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

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.."

>  *
>  * 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);

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".

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

>                        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);
> --
> 1.7.4.1
>
>

Otherwise, Looks good to me.

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

--
Kind regards,
Minchan Kim