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 53B133B2BA; Tue, 27 Jan 2026 00:49:31 +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=1769474971; cv=none; b=kjdH6MaAIPNMUXFVhtyC17rzOmPYe6LS8KmDEzo7bp1EBBn98D4WyYftokipoYCuZeK6pDnDTkbQSUwEUJ5xvPCVLeOeEMt0gFtOdc/hkLP3jb9xtgGVofiRgL+UxpCumsUNalAy635BMvsQovSO9xtjeE0M4caiwk8PdnJwLAM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769474971; c=relaxed/simple; bh=YxPRChELy5zFQlHPCSp/HP6hX6wcOkeilJU/ZU2JF6Q=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=VH/n+Pfyjd36Ka5NYMI8vPk9LQ7LFwDnV9JkHCvzFvUbWiBqSaB96vaMeLxny9XCr47M3Qo5NcQgFxxYoXss7XpzUhlKfdSZeZRmw74nTVxJpJWtDNARX6eU8ObaAg0tsdLolrnLXd6kYmCAqgeNhlLiM2UIzkmU1odRwE1FkeE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=OcVtDr/q; 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="OcVtDr/q" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CA89FC116C6; Tue, 27 Jan 2026 00:49:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1769474971; bh=YxPRChELy5zFQlHPCSp/HP6hX6wcOkeilJU/ZU2JF6Q=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=OcVtDr/qj2olVgPGFRtCcZwaOehVSAOJ3a3KspOklSaiLtOMhtKc2ZwfsrrYQJ4GL wk7Wxwga3PXJknSqt5LNs/FAxoqHX51RICx8n8Bp0LLG1gD9L0WjpIe16CjyXBMiPS hzquXAiT5gtJ9zr39MIk0+/9CqDErUSzVC0fx7UDQIWeGVr6VK9FobSWoazW92ZZTH T4z6UaYaTOKpeiZIX9IHTdTobcF/T16FevALVKKGTljJlWaIkb/IBIWLvkcxBXEQc4 e0dpn2hYZhPYK3OBOcJw/dE7o4jRph3fH3lo4OuXZH1nyKiSlnSku/uOM8GzR0kCMm ob+ARstMY0KLw== Message-ID: <2d6276fca349357f56733268681424b0de5179f7.camel@kernel.org> Subject: Re: [RFC v1] man/man2/close.2: CAVEATS: Document divergence from POSIX.1-2024 From: Jeff Layton To: Trevor Gross , Jan Kara , The 8472 Cc: Zack Weinberg , Rich Felker , Alejandro Colomar , Vincent Lefevre , Alexander Viro , Christian Brauner , linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, GNU libc development Date: Mon, 26 Jan 2026 19:49:28 -0500 In-Reply-To: References: <20260120174659.GE6263@brightrain.aerifal.cx> <1ec25e49-841e-4b04-911d-66e3b9ff4471@app.fastmail.com> <0f60995f-370f-4c2d-aaa6-731716657f9d@infinite-source.de> <20260124213934.GI6263@brightrain.aerifal.cx> <7654b75b-6697-4aad-93fc-29fa9b734bdb@infinite-source.de> <72100ec4b1ec0e77623bfdb927746dddc77ed116.camel@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; 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text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.58.2 (3.58.2-1.fc43) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Mon, 2026-01-26 at 17:01 -0600, Trevor Gross wrote: > On Mon Jan 26, 2026 at 10:43 AM CST, Jeff Layton wrote: > > On Mon, 2026-01-26 at 16:56 +0100, Jan Kara wrote: > > > On Mon 26-01-26 14:53:12, The 8472 wrote: > > > > On 26/01/2026 13:15, Jan Kara wrote: > > > > > On Sun 25-01-26 10:37:01, Zack Weinberg wrote: > > > > > > On Sat, Jan 24, 2026, at 4:57 PM, The 8472 wrote: > > > > > > > > [QUERY: Do delayed errors ever happen in any of these= situations? > > > > > > > >=20 > > > > > > > > - The fd is not the last reference to the open fil= e description > > > > > > > >=20 > > > > > > > > - The OFD was opened with O_RDONLY > > > > > > > >=20 > > > > > > > > - The OFD was opened with O_RDWR but has never act= ually > > > > > > > > been written to > > > > > > > >=20 > > > > > > > > - No data has been written to the OFD since the la= st call to > > > > > > > > fsync() for that OFD > > > > > > > >=20 > > > > > > > > - No data has been written to the OFD since the la= st call to > > > > > > > > fdatasync() for that OFD > > > > > > > >=20 > > > > > > > > If we can give some guidance about when people don= =E2=80=99t need to > > > > > > > > worry about delayed errors, it would be helpful.] > > > > > >=20 > > > > > > In particular, I really hope delayed errors *aren=E2=80=99t* ev= er reported > > > > > > when you close a file descriptor that *isn=E2=80=99t* the last = reference > > > > > > to its open file description, because the thread-safe way to cl= ose > > > > > > stdout without losing write errors[2] depends on that not happe= ning. > > > > >=20 > > > > > So I've checked and in Linux ->flush callback for the file is cal= led > > > > > whenever you close a file descriptor (regardless whether there ar= e other > > > > > file descriptors pointing to the same file description) so it's u= pto > > > > > filesystem implementation what it decides to do and which error i= t will > > > > > return... Checking the implementations e.g. FUSE and NFS *will* r= eturn > > > > > delayed writeback errors on *first* descriptor close even if ther= e are > > > > > other still open descriptors for the description AFAICS. > >=20 > > ...and I really wish they _didn't_. > >=20 > > Reporting a writeback error on close is not particularly useful. Most > > filesystems don't require you to write back all data on a close(). A > > successful close() on those just means that no error has happened yet. > >=20 > > Any application that cares about writeback errors needs to fsync(), > > full stop. >=20 > Is there a good middle ground solution here? >=20 > It seems reasonable that an application may want to have different > handling for errors expected during normal operation, such as temporary > network failure with NFS, compared to more catastrophic things like > failure to write to disk. The reason cited around [1] for avoiding fsync > is that it comes with a cost that, for many applications, may not be > worth it unless you are dealing with NFS. >=20 > I was wondering if it could be worth a new fnctl that provides this kind > of "best effort" error checking behavior without having the strict > requirements of fsync. In effect, to report the errors that you might > currently get at close() before actually calling close() and losing the > fd. >=20 For a long-held fd, I can see the appeal: spray writes at it and just check occasionally (without blocking) that nothing has gone wrong. Maybe when things are idle, you fsync(). A new fcntl(..., F_CHECKERR, ...) command that does a file_check_and_advance_wb_err() on the fd and reports the result would be pretty straightforward. Would that be helpful for your use-case? This would be like a non- blocking fsync that just reports whether an error has occurred since the last F_CHECKERR or fsync(). > Alternatively, it would be interesting to have a deferred fsync() that > schedules a nonblocking sync event that can be polled for completion/ > errors, with flags to indicate immediate sync or allow automatic syncing > as needed. But there is probably a better alternative to this > complexity. >=20 > [1]: https://github.com/rust-lang/libs-team/issues/705 Aside from the polling, I suppose you could effectively do this with io_uring. I'm pretty sure you can issue an fsync() or sync_file_range() that way, but I think it just ends up blocking a kernel thread until writeback is done. We've had people ask for a non-blocking fsync before. Maybe it's time to get serious about adding one. What would such a thing look like? It would be pretty simple to add a new fcntl(..., F_DATAWRITE) command that kicks off writeback a'la filemap_fdatawrite(). Then add fcntl(..., F_WB_CHECK): That could do a non-blocking version of filemap_fdatawait(), and return whether any folios are still under writeback. If there is a writeback error, it can return that instead. The catch of course is that a polling mechanism like this could easily livelock. If there is a lot of memory pressure, it might always return that something is still under writeback, no matter how often you hammer F_CHECKERR. Maybe that's ok? You can always issue a blocking fsync() if you really need to know draw a line in the sand. --=20 Jeff Layton