From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 13F1B2D541E; Fri, 14 Nov 2025 17:21:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763140871; cv=none; b=JVjavL8C61pAYN5JQSyzOHm5Xr7exMckmWEuOR0TXf+TGuVpgvdl6aUqNtl70dr6pT/YiNMYgHpMYSXThapMJrukRrho3vYaTT0n4QRX3KmjktuRIf6DG4cft90Wcktc9p53Scu2CHmZdvCcnwNR7uzo/4psvEOEC6p5pcWmRhc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763140871; c=relaxed/simple; bh=o4wUBZrrP4HjuCLrYyd45VSZhHaqi9XvuTvsSmX9Ms4=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=JJ5ZZcnjuEwyyEalZakQuKJbTk+ysLiT4NGt86svzsSpbQJE/PfS+pn3ptkywWiYqR6XT+VOaE9VOBcLBpMT4S/f7nzs6jY6HZ8JaQdes+O6C7B59s5GZ4n3Q+IxfkyYCpqI+tS7loVjUz1DctLkPjfIi0t9J5cMkSiEQvpWaSQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=BAt9J/6c; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="BAt9J/6c" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6B262C113D0; Fri, 14 Nov 2025 17:21:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1763140870; bh=o4wUBZrrP4HjuCLrYyd45VSZhHaqi9XvuTvsSmX9Ms4=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=BAt9J/6c9+owmHQTyiKI3qK8glE/UKpqTTBzZRk/lpUV2DeTZkHVBV8HyyMOb1nqL zA98n/JEUCU0odjvTfPSDyNRdCpAHfPXr4d4e9qHWPGOsC+6X7mASI4vDwgzJs4cih 2EZKyAvtmYcmY7Y+ACzUfzXP0LeKkDd4cap4pVwIGT50+GBxBf1edz8DQQvz9yZP/K trZ7Ritt3aUvM8SPIh0NnQcgS3+8L2CkWEKplJw45lrrqVyLQSs2fnzZil5toa1C3g 1CF6LsPLWYERdImsbr5H3Om2SujQd2e/FKEyiaZ6hxAw9okDefojw5ghEtOgDF/pWc h4QAbY3F+CdgA== Message-ID: <775371813836c06af830d9dbf6b191728636e911.camel@kernel.org> Subject: Re: re-enable IOCB_NOWAIT writes to files From: Jeff Layton To: "Darrick J. Wong" Cc: Christoph Hellwig , Christian Brauner , Al Viro , David Sterba , Jan Kara , Mike Marshall , Martin Brandenburg , Carlos Maiolino , Stefan Roesch , linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org, gfs2@lists.linux.dev, io-uring@vger.kernel.org, devel@lists.orangefs.org, linux-unionfs@vger.kernel.org, linux-mtd@lists.infradead.org, linux-xfs@vger.kernel.org, linux-nfs@vger.kernel.org Date: Fri, 14 Nov 2025 12:21:08 -0500 In-Reply-To: <20251114170129.GI196370@frogsfrogsfrogs> References: <20251114062642.1524837-1-hch@lst.de> <20251114170129.GI196370@frogsfrogsfrogs> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 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Wong wrote: > 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, > > >=20 > > > commit 66fa3cedf16a ("fs: Add async write file modification handling.= ") > > > effectively disabled IOCB_NOWAIT writes as timestamp updates currentl= y > > > always require blocking, and the modern timestamp resolution means we > > > always update timestamps. This leads to a lot of context switches fr= om > > > applications using io_uring to submit file writes, making it often wo= rse > > > than using the legacy aio code that is not using IOCB_NOWAIT. > > >=20 > > > This series allows non-blocking updates for lazytime if the file syst= em > > > supports it, and adds that support for XFS. > > >=20 > > > It also fixes the layering bypass in btrfs when updating timestamps o= n > > > 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 st= ill > > > not sure that nfsd usage is fully correct for all file systems, as on= ly > > > XFS explicitly supports FMODE_NOCMTIME, but at least the generic code > > > does the right thing now. > > >=20 > > > Diffstat: > > > Documentation/filesystems/locking.rst | 2=20 > > > 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=20 > > > 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(-) > >=20 > > 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: > >=20 > > Reviewed-by: Jeff Layton > >=20 > > 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. > >=20 > > 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. > >=20 > > Am I missing some subtlety? >=20 > In exchangerange specifically? >=20 > 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. >=20 > Also there's no way that we can do nonblocking exchangerange so a NOWAIT > flag wouldn't be much help here anyway. >=20 > (I hope that answers your question) >=20 >=20 Christoph mentioned nfsd might be doing something wrong, which is my main interest here. nfsd doesn't have a way to expose exchangerange functionality right now, but if it did then it seems like that would just work too. HCH says: > Nothing requires file_update_time / file_modified_flags are helpers > that a file system may or may not call. I've not done an audit > if everyone actually uses them. I'll have to think about how to efficiently audit that. The good news is that nfsd really only cares about the write() and page_mkwrite() codepaths. For other activity, the delegation will be broken and recalled. --=20 Jeff Layton