linux-kernel - Re: re-enable IOCB_NOWAIT writes to files

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Message-ID: <20251114170129.GI196370@frogsfrogsfrogs>
Date: Fri, 14 Nov 2025 09:01:29 -0800
From: "Darrick J. Wong" <djwong@...nel.org>
To: Jeff Layton <jlayton@...nel.org>
Cc: Christoph Hellwig <hch@....de>, Christian Brauner <brauner@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>, David Sterba <dsterba@...e.com>,
	Jan Kara <jack@...e.cz>, Mike Marshall <hubcap@...ibond.com>,
	Martin Brandenburg <martin@...ibond.com>,
	Carlos Maiolino <cem@...nel.org>, Stefan Roesch <shr@...com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	gfs2@...ts.linux.dev, io-uring@...r.kernel.org,
	devel@...ts.orangefs.org, linux-unionfs@...r.kernel.org,
	linux-mtd@...ts.infradead.org, linux-xfs@...r.kernel.org,
	linux-nfs@...r.kernel.org
Subject: Re: re-enable IOCB_NOWAIT writes to files

On Fri, Nov 14, 2025 at 09:04:58AM -0500, Jeff Layton wrote:
> On Fri, 2025-11-14 at 07:26 +0100, Christoph Hellwig wrote:
> > Hi all,
> > 
> > commit 66fa3cedf16a ("fs: Add async write file modification handling.")
> > effectively disabled IOCB_NOWAIT writes as timestamp updates currently
> > always require blocking, and the modern timestamp resolution means we
> > always update timestamps.  This leads to a lot of context switches from
> > applications using io_uring to submit file writes, making it often worse
> > than using the legacy aio code that is not using IOCB_NOWAIT.
> > 
> > This series allows non-blocking updates for lazytime if the file system
> > supports it, and adds that support for XFS.
> > 
> > It also fixes the layering bypass in btrfs when updating timestamps on
> > device files for devices removed from btrfs usage, and FMODE_NOCMTIME
> > handling in the VFS now that nfsd started using it.  Note that I'm still
> > not sure that nfsd usage is fully correct for all file systems, as only
> > XFS explicitly supports FMODE_NOCMTIME, but at least the generic code
> > does the right thing now.
> > 
> > Diffstat:
> >  Documentation/filesystems/locking.rst |    2 
> >  Documentation/filesystems/vfs.rst     |    6 ++
> >  fs/btrfs/inode.c                      |    3 +
> >  fs/btrfs/volumes.c                    |   11 +--
> >  fs/fat/misc.c                         |    3 +
> >  fs/fs-writeback.c                     |   53 ++++++++++++++----
> >  fs/gfs2/inode.c                       |    6 +-
> >  fs/inode.c                            |  100 +++++++++++-----------------------
> >  fs/internal.h                         |    3 -
> >  fs/orangefs/inode.c                   |    7 ++
> >  fs/overlayfs/inode.c                  |    3 +
> >  fs/sync.c                             |    4 -
> >  fs/ubifs/file.c                       |    9 +--
> >  fs/utimes.c                           |    1 
> >  fs/xfs/xfs_iops.c                     |   29 ++++++++-
> >  fs/xfs/xfs_super.c                    |   29 ---------
> >  include/linux/fs.h                    |   17 +++--
> >  include/trace/events/writeback.h      |    6 --
> >  18 files changed, 152 insertions(+), 140 deletions(-)
> 
> This all looks pretty reasonable to me. There are a few changelog and
> subject line typos, but the code changes look fine. You can add:
> 
> Reviewed-by: Jeff Layton <jlayton@...nel.org>
> 
> As far as nfsd's usage of FMODE_NOCMTIME, it looks OK to me. That's
> implemented today by the check in file_modified_flags(), which is
> generic and should work across filesystems.
> 
> The main exception is xfs_exchange_range() which has some special
> handling for it, but nfsd doesn't use that functionality so that
> shouldn't be an issue.
> 
> Am I missing some subtlety?

In exchangerange specifically?

The FMODE_NOCMTIME checks in xfs_exchange_range exist to tell the
exchange-range code to update cmtime, but only if it decides to actually
go through with the mapping exchange.  Since the mapping exchange
requires a transaction anyway, it's cheap to bundle in timestamp
updates.

Also there's no way that we can do nonblocking exchangerange so a NOWAIT
flag wouldn't be much help here anyway.

(I hope that answers your question)

--D