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@xxxxxxxxx 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@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
