From: "NeilBrown" <neilb@suse.de>
To: "David Laight" <David.Laight@ACULAB.COM>
Cc: "Al Viro" <viro@zeniv.linux.org.uk>,
"Chuck Lever" <chuck.lever@oracle.com>,
"Christian Brauner" <brauner@kernel.org>,
"Jens Axboe" <axboe@kernel.dk>, "Oleg Nesterov" <oleg@redhat.com>,
"Jeff Layton" <jlayton@kernel.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Subject: RE: [PATCH 1/3] nfsd: use __fput_sync() to avoid delayed closing of files.
Date: Sat, 16 Dec 2023 09:36:25 +1100 [thread overview]
Message-ID: <170267978502.12910.3767924819236993323@noble.neil.brown.name> (raw)
In-Reply-To: <ac74bdb82e114d71b26864fe51f6433b@AcuMS.aculab.com>
On Sat, 16 Dec 2023, David Laight wrote:
> ...
> > > PS: put it that way - I can buy "nfsd is doing that only to regular
> > > files and not on an arbitrary filesystem, at that; having the thread
> > > wait on that sucker is not going to cause too much trouble"; I do *not*
> > > buy turning it into a thing usable outside of a very narrow set of
> > > circumstances.
> > >
> >
> > Can you say more about "not on an arbitrary filesystem" ?
> > I guess you means that procfs and/or sysfs might be problematic as may
> > similar virtual filesystems (nfsd maybe).
>
> Can nfs export an ext4 fs that is on a loopback mount on a file
> that is remotely nfs (or other) mounted?
Sure. There is no reason this would cause a problem.
If the nfs mount were also a loopback mount, that might be interesting.
i.e. You have a local filesystem, containing a file with a filesystem
image.
You nfs-export that local filesystem, and nfs mount it on the same host.
Then you loop-mount that file in the nfs-mounted filesystem. Now you
are getting into uncharted waters. I've testing loop-back NFS mounting
and assure that it works. I haven't tried the double-loop.
But if that caused problem, I though it would be fput. It would be
fsync or writeback which causes the problem.
>
> As soon as you get loops like that you might find that fput() starts
> being problematic.
When calling fput on a regular file there are, I think, only two problem
areas. One is that the fput might lead to a lazy-filesystem unmount
completing. That only applies to MNT_INTERNAL filesystems, and they are
unlikely to be exported (probably it's impossible, but I haven't
checked).
The other is synchronous (or even async) IO in the filesystem code,
maybe completing an unlink or a truncate. This is no worse than any
other synchronous IO that nfsd does.
Thanks,
NeilBrown
>
> I'm also sure I remember that nfs wasn't supposed to respond to a write
> until it had issued the actual disk write - but maybe no one do that
> any more because it really is too slow.
> (Especially if the 'disk' is a USB stick.)
>
> David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>
next prev parent reply other threads:[~2023-12-15 22:36 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-08 3:27 [PATCH 0/3] nfsd: fully close all files in the nfsd threads NeilBrown
2023-12-08 3:27 ` [PATCH 1/3] nfsd: use __fput_sync() to avoid delayed closing of files NeilBrown
2023-12-08 15:01 ` Chuck Lever
2023-12-10 22:47 ` NeilBrown
2023-12-11 19:01 ` Chuck Lever
2023-12-11 22:04 ` NeilBrown
2023-12-12 16:17 ` Chuck Lever
2023-12-11 19:11 ` Al Viro
2023-12-11 22:23 ` NeilBrown
2023-12-11 23:13 ` Al Viro
2023-12-11 23:21 ` Al Viro
2023-12-13 0:28 ` NeilBrown
2023-12-15 18:27 ` David Laight
2023-12-15 19:35 ` Chuck Lever III
2023-12-15 22:36 ` NeilBrown [this message]
2023-12-15 18:28 ` David Laight
2023-12-16 1:50 ` Dave Chinner
2023-12-08 3:27 ` [PATCH 2/3] nfsd: Don't leave work of closing files to a work queue NeilBrown
2023-12-08 3:27 ` [PATCH 3/3] VFS: don't export flush_delayed_fput() NeilBrown
2023-12-08 11:40 ` [PATCH 0/3] nfsd: fully close all files in the nfsd threads Jeff Layton
2023-12-08 14:33 ` Jens Axboe
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