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

Shaohua Li
2011-06-24T05:57:59Z

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@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
> *
> * 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