On Wed, Aug 14, 2013 at 11:18 PM, David Lang [off-list ref] wrote:

On Wed, 14 Aug 2013, Andy Lutomirski wrote:

quoted

quoted

The big problem with this approach is that not doing the
timestamp update on page faults is going to break the inode change
version counting because for ext4, btrfs and XFS it takes a
transaction to bump that counter. NFS needs to know the moment a
file is changed in memory, not when it is written to disk. Also, NFS
requires the change to the counter to be persistent over server
failures, so it needs to be changed as part of a transaction....

NFS can do whatever it wants, although I suspect that even NFS can get
away with deferring cmtime updates.

NFS already has to do syncs to make sure the data is safe on disk, have a
flag that NFS can use to make the ctime safe, everyone else can get the
performance improvement and NFS can have it's slow-but-safe approach.

I don't see the current code that updates times for NFS.  I'm not
planning on making any changes that'll affect NFS at all (i.e. I don't
think any flag will be needed), but I'd be more confident if I
understand why it worked in the first place.

(For filesystems that provide page_mkwrite, there hasn't been a
file_update_time call in the core code for several kernel versions.)

--Andy

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