Re: [PATCH 00/67] fscache: Rewrite index API and management system

Jeff Layton
2021-10-21T23:43:37Z

On Thu, 2021-10-21 at 18:15 -0500, Steve French wrote:
> On Thu, Oct 21, 2021 at 5:21 PM Omar Sandoval <osandov@osandov.com> wrote:
> >
> > On Mon, Oct 18, 2021 at 03:50:15PM +0100, David Howells wrote:
> > However, with the advent of the tmpfile capacity in the VFS, an opportunity
> > arises to do invalidation much more easily, without having to wait for I/O
> > that's actually in progress: Cachefiles can simply cut over its file
> > pointer for the backing object attached to a cookie and abandon the
> > in-progress I/O, dismissing it upon completion.
>
> Have changes been made to O_TMPFILE? It is problematic for network filesystems
> because it is not an atomic operation, and would be great if it were possible
> to create a tmpfile and open it atomically (at the file system level).
>
> Currently it results in creating a tmpfile (which results in
> opencreate then close)
> immediately followed by reopening the tmpfile which is somewhat counter to
> the whole idea of a tmpfile (ie that it is deleted when closed) since
> the syscall results
> in two opens ie open(create)/close/open/close
>
>

In this case, O_TMPFILE is being used on the cachefiles backing store,
and that usually isn't deployed on a netfs. That said, Steve does have a
good point...

What happens if you do end up without O_TMPFILE support on the backing
store? Probably just opting to not cache in that case is fine. Does
cachefiles just shut down in that situation?
--
Jeff Layton <jlayton@kernel.org>