Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain timestamp handing

Linus Torvalds
2023-10-24T00:19:01Z

On Mon, 23 Oct 2023 at 13:26, Dave Chinner <david@fromorbit.com> wrote:
>
> The problem is the first read request after a modification has been
> made. That is causing relatime to see mtime > atime and triggering
> an atime update. XFS sees this, does an atime update, and in
> committing that persistent inode metadata update, it calls
> inode_maybe_inc_iversion(force = false) to check if an iversion
> update is necessary. The VFS sees I_VERSION_QUERIED, and so it bumps
> i_version and tells XFS to persist it.

Could we perhaps just have a mode where we don't increment i_version
for just atime updates?

Maybe we don't even need a mode, and could just decide that atime
updates aren't i_version updates at all?

Yes, yes, it's obviously technically a "inode modification", but does
anybody actually *want* atime updates with no actual other changes to
be version events?

Or maybe i_version can update, but callers of getattr() could have two
bits for that STATX_CHANGE_COOKIE, one for "I care about atime" and
one for others, and we'd pass that down to inode_query_version, and
we'd have a I_VERSION_QUERIED and a I_VERSION_QUERIED_STRICT, and the
"I care about atime" case ould set the strict one.

Then inode_maybe_inc_iversion() could - for atome updates - skip the
version update *unless* it sees that I_VERSION_QUERIED_STRICT bit.

Does that sound sane to people?

Because it does sound completely insane to me to say "inode changed"
and have a cache invalidation just for an atime update.

Linus