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 9E08517A2FC; Wed, 6 May 2026 06:11:40 +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=1778047900; cv=none; b=Wi7n0dfZ8meJqmvVvugffsxS62mXbt6yeqWYfeALbZ7Fh7QVsOXd2R9zf10DbXliiYEkrk1Pj38kPa8hFKr06g2P0umNp933d45cYw3AVc9VCXqmaTMac3ataNiRcA+DCxRNMhE/gcmC8ZiMYnHO5WJABJpx8b5PR7SjMS1A57c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778047900; c=relaxed/simple; bh=D3OucNZZnd3lVVGurhT1KSeuP2zmZt8RbgVk9MVPtuo=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=ddqS4DXL4LBVD3r1uizoGDQi+xeC6r2n+4tIVdi3lvyRZ6Y69/mv/ZHZWc7DJENSrzRIWOyUzHIkLML/5B2bFIZk+MM6eTDCVDohw0/co6hzAFlZskVatq2G1ekwL8MOd0mFJF6v/u407A6+xqk0KOSw+7QSZTdcx+l6BWTtpwA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=iGL0A+Zd; 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="iGL0A+Zd" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5EF25C2BCB8; Wed, 6 May 2026 06:11:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778047900; bh=D3OucNZZnd3lVVGurhT1KSeuP2zmZt8RbgVk9MVPtuo=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=iGL0A+Zd1cdlEKkVwDkysI1zCOSsteoUVdhqOcnexCQIM9ftneApO5TMB5pz95K/d mmhe7uID6EI+abKBOGOSm95ZpSUWr44iLrUIV69Yh04o47B0SCu2SubKZbe8r8qHBm AXi/M9LdRH4QZOMLBlr12ClofkYPQ4Wlc/rBIR+o4yedt2JUltHmSbI3FsBsWfvEeJ GJmHYX9jZIG8BrfkTR0EMnAGwa8TND0CJ7V5BVJHl901gFfqdSZJwe+QVyKpYYRR8a q+gqlNHhy3o5mSqwxW0kwvkguUAAYpS0pAiXT8ZmErq7SfAkZ4jcy22k0cfIn5/1Gs V9Yv993b+flLg== Message-ID: <9e48229614786e0c2e92bb6a2dd3269868f160d0.camel@kernel.org> Subject: Re: [DISCUSSION] Preventing ENOSPC/EDQUOT writeback errors on network filesystems From: Jeff Layton To: Piyush Sachdeva , linux-fsdevel@vger.kernel.org, linux-cifs@vger.kernel.org, linux-nfs@vger.kernel.org, netfs@lists.linux.dev Cc: sprasad@microsoft.com, linux-kernel@vger.kernel.org, sfrench@samba.org Date: Wed, 06 May 2026 08:11:35 +0200 In-Reply-To: References: Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 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plenty of discussions on how to handle writeback errors f= or > network filesystems, but most have focused on error reporting after the f= act. > I'd like to start a discussion around preventing writeback errors specifi= cally > ENOSPC and EDQUOT, before they cause silent data loss. >=20 > The problem: > With buffered writes on network filesystems (cifs, nfs, etc.), the write(= ) > syscall copies data into the page cache and returns success immediately. = The > actual upload to the server happens later during writeback. If the server= is > out of space at that point, the write fails with ENOSPC. The netfs/writeb= ack > layer records this error via mapping_set_error(), but critically the foli= o's > writeback flag is cleared and the page is now clean. Under memory pressur= e, the > VM can reclaim these clean pages, permanently losing data that the applic= ation > believes was successfully written. Meanwhile, i_size has already been upd= ated > to reflect the new file size. So stat() shows a file size inclusive of th= e data > that was never persisted. Another inconsistency here is that total free s= pace > hasn't been modified for the file system on the server, leading to incorr= ect > values in statfs() output from the client's pov (assuming statfs() calls = go > to the server). > To illustrate with real-world scenarios: >=20 > - A user or application can keep issuing writes to an fd well beyond the > available space, since buffered writes return success as soon as data i= s > copied to the page cache. A significant amount of data, exceeding the > available quota can accumulate before fsync() is called, at which point > critical data loss is nearly certain. >=20 > - A malicious user can exploit this to keep resources pinned and memory > oversubscribed, impacting other applications. >=20 > The error is technically observable: fsync() will return it, and close() > surfaces it through the flush callback. But in practice, many application= s > check neither, and the POSIX "just call fsync()" answer isn't satisfying = for > users who lose data silently. >=20 Yet, it is the only real answer we have. This is just a fundamental issue with buffered writes and delayed writeback. Either you flush the data to stable storage now, or you have to do it later. If you do it later, then it can still fail for all sorts of reasons. > Local filesystems largely avoid this because they can check available spa= ce > synchronously in write_begin() and fail the write() syscall directly. Net= work > filesystems can't do this cheaply =E2=80=94 a round-trip per write to che= ck server > space would negate the benefits of buffered I/O. >=20 > Through recent development, netfs is becoming a central layer for network > filesystem I/O. It already has retry logic for transient failures (EAGAIN= , > ECONNABORTED), but ENOSPC/EDQUOT remain hard failures. This affects every > network filesystem using buffered writes. >=20 > I am curious to know if NFS has a solution to this and what the approach = is > towards this specific problem by NFS community? >=20 > This problem is worth solving for all network filesystems. I have a few > thoughts on approaches, combining cached statfs() output with > fallocate()-style pre-allocation on the write path: >=20 > 1. Pre-allocate space on the server before writing to the page cache, > analogous to fallocate() on the write path. This guarantees server-sid= e > space for page cache data. >=20 > 2. Since per-write fallocate() calls require a server round-trip, effecti= vely > negating the benefit of buffered I/O. Use cached statfs() output to ga= te > when pre-allocation is triggered. For example, once free space drops b= elow > 20% of total space, enable fallocate() on the write path. Otherwise, l= et > writes proceed as normal. >=20 > 3. Handle refresh and synchronization of the cached statfs() data separat= ely > to avoid staleness. >=20 > I'd appreciate feedback from the community on viable approaches. NFSv4.2 does have an ALLOCATE operation: https://datatracker.ietf.org/doc/html/rfc7862#section-15.1 ...and such an operation could (in principle) precede WRITE in a compound, but that doesn't really help you. By the time we're issuing RPCs to the server, the client application has already finished its writes and moved on. For applications that want to avoid ENOSPC/EDQUOT, the best thing they could do is call fallocate() themselves to ensure that the space exists. With a sufficiently recent NFS client and server, that should DTRT. --=20 Jeff Layton