Re: [PATCH v3 0/5] Rework mtime and ctime updates on mmaped writes

[Jan Kara <jack@suse.cz>]

On Thu 22-08-13 17:03:16, Andy Lutomirski wrote:
> Writes via mmap currently update mtime and ctime in ->page_mkwrite.
> This hurts both throughput and latency.  In workloads that dirty a
> large number of mmapped pages, ->page_mkwrite can be hot and
> file_update_time is slow and scales poorly.  Updating timestamps can
> also sleep, which hurts latency for real-time workloads.
> 
> This is also a correctness issue.  SuS says:
> 
>     The st_ctime and st_mtime fields of a file that is mapped with
>     MAP_SHARED and PROT_WRITE, will be marked for update at some point
>     in the interval between a write reference to the mapped region and
>     the next call to msync() with MS_ASYNC or MS_SYNC for that portion
>     of the file by any process. If there is no such call, these fields
>     may be marked for update at any time after a write reference if
>     the underlying file is modified as a result.
> 
> Currently, if the same mmapped page is written twice, the timestamp
> may not be update at all after the second write, whereas SuS (and
> anything using timestamps to invalidate caches, backup data, etc.)
> would expect the timestamp to eventually be updated.
> 
> This patchset attempts to fix both issues at once.  It adds a new
> address_space flag AS_CMTIME that is set atomically whenever the
> system transfers a pte dirty bit to a struct page backed by the
> address_space.  This can happen with various locks held and when low
> on memory.
> 
> Later on, a_ops.update_cmtime_deferred is called to tell the FS to
> update cmtime due to a previous mmapped write.
> 
> The core changes have no effect on unmodified filesystems.  To opt in,
> a filesystem should implement .update_cmtime_deferred (most likely by
> using generic_update_cmtime_deferred) and must call either
> mapping_flush_cmtime or mapping_test_clear_cmtime in .writepages.
> Filesystems should avoid updating timestamps in ->page_mkwrite.
> 
> The reason that this is not completely automatic is that filesystems
> without backing stores do not really fit in to this model.
> Eventually, someone can add support.
> 
> I've converted ext4, xfs, and btrfs.  Converting most other
> filesystems should be straightforward.
> 
> I wrote an xfstest for this.  ext4, xfs, and btrfs pass.  It's here:
> 
> https://github.com/amluto/xfstests/commit/5fbb72ac799cc44a9c4c6d3919f00a479202c899
> 
> This series is pullable from:
> 
> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=mmap_mtime/patch_v4
  As a general note, I think you should CC linux-mm@kvack.org on this
series so that mm guys are more likely to notice it. Since the patches
touch mm you should probably get some opinions from them...

								Honza
> 
> Changes from v3:
>  - The new address space op is now called update_cmtime_deferred.
>    Callers take care of protection from fs freezing and checking
>    AS_CMTIME.  I fixed a deadlock in the freezer interaction.
>  - Block plugs should be handled better.
>  - Fixed an infinite loop in msync(MS_ASYNC).
>  - Converted xfs and btrfs.
>  - Misc minor cleanups.
>  - Fixed a corner case: reclaim or migration could have cleaned all
>    pages without updating cmtime.
> 
> Changes from v2:
>  - The core code now interacts with filesystems only through
>    address_space ops, so there should be fewer layering issues.
>  - MS_ASYNC is handled correctly.
> 
> Changes from v1:
>  - inode_update_time_writable now locks against the fs freezer.
>  - Minor cleanups.
>  - Major changelog improvements.
> 
> Andy Lutomirski (7):
>   mm: Track mappings that have been written via ptes
>   fs: Add inode_update_time_writable
>   mm: Allow filesystems to defer cmtime updates
>   mm: Scan for dirty ptes and update cmtime on MS_ASYNC
>   ext4: Defer mmap cmtime updates
>   btrfs: Defer mmap cmtime updates
>   xfs: Defer mmap cmtime updates
> 
>  fs/btrfs/extent_io.c      |  1 +
>  fs/btrfs/inode.c          | 32 +++++++++---------
>  fs/buffer.c               |  7 ----
>  fs/ext4/inode.c           | 11 +++++--
>  fs/inode.c                | 64 +++++++++++++++++++++++++++---------
>  fs/xfs/xfs_aops.c         |  1 +
>  include/linux/fs.h        |  9 +++++
>  include/linux/pagemap.h   | 22 +++++++++++++
>  include/linux/writeback.h |  1 +
>  mm/memory.c               |  7 +++-
>  mm/migrate.c              |  2 ++
>  mm/mmap.c                 |  6 +++-
>  mm/msync.c                | 84 ++++++++++++++++++++++++++++++++++++++++-------
>  mm/page-writeback.c       | 53 +++++++++++++++++++++++++++++-
>  mm/rmap.c                 | 27 +++++++++++++--
>  mm/vmscan.c               |  1 +
>  16 files changed, 272 insertions(+), 56 deletions(-)
> 
> -- 
> 1.8.3.1
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR