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 A70A536CE03; Fri, 29 May 2026 17:03:32 +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=1780074213; cv=none; b=ASvpYgio0yc35iXAEDl1EXNyir/2muKcF7ugNHOxY9LG6MdaKWkyeqSkDKP3AuFUdMVv/Msy3RwVC1eQE+n1kJrE4ZjmAJj4GLrV2KBDFbGY3Pn4CjhUzq9bhFxsxfws7B6/LpOVMHWCsEjmuZjEFyrkfISkv0q/yNca+twdd4w= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780074213; c=relaxed/simple; bh=GI9gLjZGbP1zI4FQZhM5/EPXzwCN5+TXNGKETmn7TyU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=B44bAI5RhenhaZ6oOWKhHeqnCgq6t2ajciEUxLmhQsbC9wlCS85X1pJUflU8t/NkUUZNwqPVzmmVZxuJNjlw6Yoq2MqAFl7g37Pc/vO0ijO3A+VFQG+AGO9qP3vWHKgeUm8ham5SOSSGrIjOL3nhAKwlTKwZXbWaHaalGLis300= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Q8Wi1P0R; 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="Q8Wi1P0R" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3639E1F00893; Fri, 29 May 2026 17:03:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780074212; bh=0tKM/Plje/NtNSHtGHaAFyrlxtQPKWNa20U5O8qQp08=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Q8Wi1P0Rx9tMDIAVUB56HGba2kkHHH2+aQUJW4uHoeZKhInwIVBb5YrIqiyEwKStL POnY5Iwr5aOnkx2bLcw7kFOWGHJ4eaAIfSia+w6I39MVVGnwYwj/4SzCiE3Zj1R2FM SbBDkblt21Uh4I5FHr3DSPMWh8dbVC7KSxIKU1E583rvM+zhaipYBzLWxslpUsiqcI ujMP2ZjXJYbOv3wonhIIji9iH1c9VW/zjpb1VYTdgl4BrAsMoX2KrijEkfXwbkDAtw M9EOb23CIcszZhYhoK1HQUwpNBOn8jzYuIHT8I9QEym89vUGe3O5a8ZGakoJ+CSYfC jjpbnny4njRkQ== Message-ID: <0f23fe53-3796-4102-ab62-f28fd53afaf7@kernel.org> Date: Fri, 29 May 2026 13:03:30 -0400 Precedence: bulk X-Mailing-List: linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 08/10] nfsd: fix partial-write detection in nfsd_direct_write To: Jeff Layton , 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 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> <1ab23163ff7e1b7f01326ef9f222dad25ad6a863.camel@kernel.org> From: Chuck Lever Content-Language: en-US Organization: kernel.org In-Reply-To: <1ab23163ff7e1b7f01326ef9f222dad25ad6a863.camel@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/29/26 1:01 PM, Jeff Layton wrote: > On Fri, 2026-05-29 at 12:57 -0400, Chuck Lever wrote: >> >> On Thu, May 28, 2026, at 5:55 PM, Jeff Layton wrote: >>> From: Chris Mason >>> >>> nfsd_direct_write() walks a list of write segments and, after each >>> 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: >>> >>> host_err = vfs_iocb_iter_write(file, kiocb, &segments[i].iter); >>> if (host_err < 0) >>> return host_err; >>> *cnt += host_err; >>> if (host_err < segments[i].iter.count) >>> break; /* partial write */ >>> >>> 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: >>> >>> before write_iter: iter.count == original_len >>> after write_iter: iter.count == original_len - host_err >>> >>> 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. >>> >>> Snapshot the segment's byte count before the write and compare >>> host_err against that snapshot so any short write breaks the loop. >>> >>> 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(-) >>> >>> 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 >>> svc_fh *fhp, >>> struct file *file = nf->nf_file; >>> unsigned int nsegs, i; >>> ssize_t host_err; >>> + size_t expected; >>> >>> nsegs = nfsd_write_dio_iters_init(nf, rqstp->rq_bvec, nvecs, >>> kiocb, *cnt, segments); >>> @@ -1401,11 +1402,13 @@ nfsd_direct_write(struct svc_rqst *rqstp, >>> struct svc_fh *fhp, >>> kiocb->ki_flags |= IOCB_DONTCACHE; >>> } >>> >>> + expected = iov_iter_count(&segments[i].iter); >>> + >>> host_err = vfs_iocb_iter_write(file, kiocb, &segments[i].iter); >>> if (host_err < 0) >>> return host_err; >>> *cnt += host_err; >>> - if (host_err < segments[i].iter.count) >>> + if (host_err < (ssize_t)expected) >>> break; /* partial write */ >>> } >>> >>> >>> -- >>> 2.54.0 >> >> How many filesystems can return a short write in this case? >> My impression was that only the NFS client can do that. >> > > 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. NFSD_IO_DIRECT is experimental, and doesn't make sense (to me) to use with an NFS re-export. If we can find another filesystem that might return a short write with NFSD_IO_DIRECT, I might consider this a higher priority. -- Chuck Lever