[RFC PATCH 0/4] Rework mtime and ctime updates on mmaped writes

From: Andy Lutomirski
Date: Thu Dec 20 2012 - 18:16:30 EST

Writes via mmap currently update mtime and ctime in ->page_mkwrite.
This is unfortunate from a performance and a correctness point of view.
The file times should be updated after writes, not before (so that every
write eventually results in a fresh timestamp). More importantly
(for me), ->page_mkwrite is called periodically even on mlocked pages,
and some filesystems can sleep in mark_inode_dirty.

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, whenever syncing an inode (which happens indirectly in msync)
or whenever a vma is torn down, if AS_CMTIME is set, then the file times
are updated. This happens in a context from which (I think) it's safe
to dirty inodes.

One nice property of this approach is that it requires no fs-specific
work. It's actually quite a bit simpler than I expected.

I've tested this, and mtime and ctime are updated on munmap, exit, MS_SYNC,
and fsync after writing via mmap. The times are also updated 30 seconds
after writing, all by themselves :) Lockdep has no complaints.

NB: I am not at all an expert in anything fs or pagecache related. Please
help me find things that may be wrong with these patches.

Andy Lutomirski (4):
mm: Explicitly track when the page dirty bit is transferred from a
mm: Update file times when inodes are written after mmaped writes
Remove file_update_time from all mkwrite paths
ext4: Fix an incorrect comment about i_mutex

fs/9p/vfs_file.c | 3 ---
fs/btrfs/inode.c | 4 +---
fs/buffer.c | 6 ------
fs/ceph/addr.c | 3 ---
fs/ext4/fsync.c | 2 --
fs/ext4/inode.c | 1 -
fs/gfs2/file.c | 3 ---
fs/inode.c | 37 +++++++++++++++++++++++++++++++++++++
fs/nilfs2/file.c | 1 -
fs/sysfs/bin.c | 2 --
include/linux/fs.h | 1 +
include/linux/mm.h | 1 +
include/linux/pagemap.h | 3 +++
mm/filemap.c | 1 -
mm/memory-failure.c | 4 +---
mm/memory.c | 5 +----
mm/mmap.c | 4 ++++
mm/page-writeback.c | 42 ++++++++++++++++++++++++++++++++++++++++--
mm/rmap.c | 9 ++++++---
19 files changed, 95 insertions(+), 37 deletions(-)


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