From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 2402E36F411; Fri, 29 May 2026 17:01:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780074077; cv=none; b=AwOnlQP8drFyU/f41NQ1tiZyanC+G/45M3hm/G2lRHExtdn8JEGU80bbbIGBlePFfDK6aTSpHNr0Y/UVE2N0Ej83iKpiQRVlddzGL1JoHvVG2M++pMaHTT30RPiTiX0ilVdHueJzXNYmjF0xFnNgF20zBaXY7P+OmdEtL+zhd28= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780074077; c=relaxed/simple; bh=QAFH1kouv1ojfA2g89TIsBO779qgYlFXODZiFlcAN8c=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=hxHQMmj5fPZpKPmNz4yeM9pflDfcH79Yh9PUSlBK8bfpMT7gzhWM5cVQbU79sjo0Yc0y522KQ5idEkMcKVkJEXwnR54C7hqFfQu3bF0srMlWzXpxWLQKBUwwjMu1yYF1UfEsTds4tZ+JTsSZUgQWf+CdcqCRaA2cSFGg8eBA4VI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=AmyBQMZt; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="AmyBQMZt" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7CFCA1F00898; Fri, 29 May 2026 17:01:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780074075; bh=leWBTF7pRLKSj1/hOsHP7ZuZLZtAnjfac7no2GVNIt4=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=AmyBQMZt8/biYcsBC6ZUUhj1HeZSPNjWbMNrBVg10pTE1CPzVBjs3R34l/Dh9L0JC NPHvXgnteLBvfPTgr+LYzCjv1ejgPk+pPNtGZiD5vCUqPNcGkDuKTCysWVw5ZwofHo OE2u/r11xX0xxZSdmCYSLGWUTwagjLMsDGz2OuVHCaqKdPoHjalbu6v22VEGAHxK3a H9Bt5MhwHSbJX8qB1A65Nvgz2RSEUcaMPKbWl3bSKLhxf19bhI9yjNFyv3xK3EuFcO R73iHF4lzXpwnW1OyWi8IDHAWH8gjw5z7/DJ6a22hkLMoC4xDanCe2+sZ5onck9ZzF ZDBJKW76Z1Hqg== Message-ID: <1ab23163ff7e1b7f01326ef9f222dad25ad6a863.camel@kernel.org> Subject: Re: [PATCH 08/10] nfsd: fix partial-write detection in nfsd_direct_write From: Jeff Layton To: Chuck Lever , Chuck Lever , NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , "J. Bruce Fields" , Scott Mayhew , Trond Myklebust , Andreas Gruenbacher , Mike Snitzer , Rick Macklem Cc: Chris Mason , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Date: Fri, 29 May 2026 13:01:12 -0400 In-Reply-To: <5ef47df4-c9f4-4b7a-a493-71be1a95ee90@app.fastmail.com> References: <20260528-nfsd-fixes-v1-0-e78708eff77d@kernel.org> <20260528-nfsd-fixes-v1-8-e78708eff77d@kernel.org> <5ef47df4-c9f4-4b7a-a493-71be1a95ee90@app.fastmail.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn 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vfs_iocb_iter_write(), tries to detect a short write so the loop can > > stop before placing the next segment at a wrong file offset: > >=20 > > host_err =3D vfs_iocb_iter_write(file, kiocb, &segments[i].iter); > > if (host_err < 0) > > return host_err; > > *cnt +=3D host_err; > > if (host_err < segments[i].iter.count) > > break; /* partial write */ > >=20 > > vfs_iocb_iter_write() runs the iter through ->write_iter(), which > > advances the iter by the number of bytes written. By the time the > > check runs, segments[i].iter.count is the residual, not the original > > request length: > >=20 > > before write_iter: iter.count =3D=3D original_len > > after write_iter: iter.count =3D=3D original_len - host_err > >=20 > > The condition then reduces to host_err < original_len - host_err, so > > the break fires only when less than half of the segment was written. > > Any short write completing between 50% and 99% of the segment slips > > through; the loop advances to the next segment with kiocb->ki_pos > > only bumped by the short amount, writing the next segment's payload > > at the wrong offset and over-reporting *cnt to the NFS client. > >=20 > > Snapshot the segment's byte count before the write and compare > > host_err against that snapshot so any short write breaks the loop. > >=20 > > Fixes: 06c5c97293e3 ("NFSD: Implement NFSD_IO_DIRECT for NFS WRITE") > > Assisted-by: kres:claude-opus-4-7 > > Signed-off-by: Chris Mason > > --- > > fs/nfsd/vfs.c | 5 ++++- > > 1 file changed, 4 insertions(+), 1 deletion(-) > >=20 > > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c > > index 980217f755b7..619f252af4d1 100644 > > --- a/fs/nfsd/vfs.c > > +++ b/fs/nfsd/vfs.c > > @@ -1380,6 +1380,7 @@ nfsd_direct_write(struct svc_rqst *rqstp, struct= =20 > > svc_fh *fhp, > > struct file *file =3D nf->nf_file; > > unsigned int nsegs, i; > > ssize_t host_err; > > + size_t expected; > >=20 > > nsegs =3D nfsd_write_dio_iters_init(nf, rqstp->rq_bvec, nvecs, > > kiocb, *cnt, segments); > > @@ -1401,11 +1402,13 @@ nfsd_direct_write(struct svc_rqst *rqstp,=20 > > struct svc_fh *fhp, > > kiocb->ki_flags |=3D IOCB_DONTCACHE; > > } > >=20 > > + expected =3D iov_iter_count(&segments[i].iter); > > + > > host_err =3D vfs_iocb_iter_write(file, kiocb, &segments[i].iter); > > if (host_err < 0) > > return host_err; > > *cnt +=3D host_err; > > - if (host_err < segments[i].iter.count) > > + if (host_err < (ssize_t)expected) > > break; /* partial write */ > > } > >=20 > >=20 > > --=20 > > 2.54.0 >=20 > How many filesystems can return a short write in this case? > My impression was that only the NFS client can do that. >=20 No idea right offhand, but NFS is exportable. Since vfs_iocb_iter_write() is allowed to return a short write, I think we have to deal with that properly here. --=20 Jeff Layton