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

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Date:   Thu, 21 Oct 2021 19:43:30 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Steve French <smfrench@...il.com>,
        Omar Sandoval <osandov@...ndov.com>
Cc:     David Howells <dhowells@...hat.com>, linux-cachefs@...hat.com,
        ceph-devel <ceph-devel@...r.kernel.org>,
        linux-afs@...ts.infradead.org,
        Anna Schumaker <anna.schumaker@...app.com>,
        linux-nfs <linux-nfs@...r.kernel.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-mm <linux-mm@...ck.org>,
        Matthew Wilcox <willy@...radead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Dave Wysochanski <dwysocha@...hat.com>,
        Marc Dionne <marc.dionne@...istor.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Shyam Prasad N <nspmangalore@...il.com>,
        Eric Van Hensbergen <ericvh@...il.com>,
        v9fs-developer@...ts.sourceforge.net,
        CIFS <linux-cifs@...r.kernel.org>,
        Latchesar Ionkov <lucho@...kov.net>,
        Steve French <sfrench@...ba.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Trond Myklebust <trondmy@...merspace.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/67] fscache: Rewrite index API and management system

On Thu, 2021-10-21 at 18:15 -0500, Steve French wrote:
> On Thu, Oct 21, 2021 at 5:21 PM Omar Sandoval <osandov@...ndov.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@...nel.org>