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    Subject[PATCH v3 0/2] fadvise: support POSIX_FADV_NOREUSE
    Date
    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_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.

    A previous proposal [4] tried to resolve this problem being less
    agressive in invalidating active pages, moving them to the inactive list
    intead of just evict them from the page cache.

    However, this approach changed completely the old behavior of
    invalidate_mapping_pages(), that is not only used by fadvise.

    The new solution maps POSIX_FADV_NOREUSE to the less-agressive page
    invalidation policy.

    With POSIX_FADV_NOREUSE active pages are moved to the tail of the
    inactive list, and pages in the inactive list are just removed from page
    cache. Pages mapped by other processes or unevictable pages are not
    touched at all.

    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_FADV_NOREUSE.
    If the page was also touched by other tasks 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.

    In conclusion, now userspace applications that want to drop some page
    cache pages can choose between the following advices:

    POSIX_FADV_DONTNEED = drop page cache if possible
    POSIX_FADV_NOREUSE = reduce page cache eligibility

    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
    - re-run md5sum zero (user activity on the working set) and measure
    the time to complete this command

    rsync has been patched with [3], using POSIX_FADV_DONTNEED in one case and
    POSIX_FADV_NOREUSE in the other case.

    Results:

    - after the backup run:
    # perf stat -e block:block_bio_queue md5sum zero

    avg elapsed time block:block_bio_queue
    rsync-dontneed 4.24s 2,072
    rsync-noreuse 2.22s 0

    [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
    [4] https://lkml.org/lkml/2011/6/23/35

    ChangeLog v2 -> v3:
    - do not change the old POSIX_FADV_DONTNEED behavior and implement the less
    aggressive page cache invalidation policy using POSIX_FADV_NOREUSE
    - fix comments in the code

    [PATCH v3 1/2] mm: introduce __invalidate_mapping_pages()
    [PATCH v3 2/2] fadvise: implement POSIX_FADV_NOREUSE

    include/linux/fs.h | 7 +++++--
    mm/fadvise.c | 11 ++++++-----
    mm/swap.c | 2 +-
    mm/truncate.c | 40 ++++++++++++++++++++++++++++++----------
    4 files changed, 42 insertions(+), 18 deletions(-)


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