From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5ACBFC432C0 for ; Tue, 19 Nov 2019 16:35:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 212C220643 for ; Tue, 19 Nov 2019 16:35:15 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="UcEI7LqK" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728095AbfKSQfO (ORCPT ); Tue, 19 Nov 2019 11:35:14 -0500 Received: from aserp2120.oracle.com ([141.146.126.78]:56726 "EHLO aserp2120.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727560AbfKSQfO (ORCPT ); Tue, 19 Nov 2019 11:35:14 -0500 Received: from pps.filterd (aserp2120.oracle.com [127.0.0.1]) by aserp2120.oracle.com (8.16.0.27/8.16.0.27) with SMTP id xAJGTNLM102172; Tue, 19 Nov 2019 16:34:58 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=date : from : to : cc : subject : message-id : references : mime-version : content-type : in-reply-to; s=corp-2019-08-05; bh=vizkMgmMDESpjnFcq/Am7MA+/C9xfAoNWHo2ZfgU82I=; b=UcEI7LqKHXXXo9aGSr+lQIPXbdgZ3oZjg/udlKPsH2VtH+C15AyuO+PFZicHsiLt/k6j d/HTRtDNjxXWJS5eUYe2LqdITMGplRB1ewL2nJSz9bwwGRi7Sk3x7YWDdKyVW5haIFjt xjXfonFvcZcpUrIkEWOI6ju6HlUGnzQGIhnVAVC8+qyuDZkxeWzJGjlyWHFnMLCTJ7Og ULXJvBTU5wJTls4dyFUXW7Mlz26Xb31+MT2LWK7qLqCynExtFvhIBaeIL00s200UeP93 0MBj5qOlbs+DMshppu4O1fFjDbV2iOe2PnN+oRPamuPzcHUg5mJCcFgL/5PowoSS2FQP Lg== Received: from aserp3020.oracle.com (aserp3020.oracle.com [141.146.126.70]) by aserp2120.oracle.com with ESMTP id 2wa92pr2xt-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 19 Nov 2019 16:34:57 +0000 Received: from pps.filterd (aserp3020.oracle.com [127.0.0.1]) by aserp3020.oracle.com (8.16.0.27/8.16.0.27) with SMTP id xAJGSBI9016064; Tue, 19 Nov 2019 16:34:57 GMT Received: from aserv0121.oracle.com (aserv0121.oracle.com [141.146.126.235]) by aserp3020.oracle.com with ESMTP id 2wcem9w4q0-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 19 Nov 2019 16:34:57 +0000 Received: from abhmp0013.oracle.com (abhmp0013.oracle.com [141.146.116.19]) by aserv0121.oracle.com (8.14.4/8.13.8) with ESMTP id xAJGYuNb007637; Tue, 19 Nov 2019 16:34:56 GMT Received: from localhost (/67.169.218.210) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Tue, 19 Nov 2019 08:34:55 -0800 Date: Tue, 19 Nov 2019 08:34:54 -0800 From: "Darrick J. Wong" To: Jan Kara Cc: linux-fsdevel@vger.kernel.org, Christoph Hellwig , Al Viro , Matthew Bobrowski , linux-ext4@vger.kernel.org, Ted Tso Subject: Re: Splice & iomap dio problems Message-ID: <20191119163454.GS6235@magnolia> References: <20191113180032.GB12013@quack2.suse.cz> <20191113184403.GM6235@magnolia> <20191119163214.GC2440@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191119163214.GC2440@quack2.suse.cz> User-Agent: Mutt/1.9.4 (2018-02-28) X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9445 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1911140001 definitions=main-1911190146 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9445 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1911140001 definitions=main-1911190146 Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Tue, Nov 19, 2019 at 05:32:14PM +0100, Jan Kara wrote: > On Wed 13-11-19 10:44:03, Darrick J. Wong wrote: > > On Wed, Nov 13, 2019 at 07:00:32PM +0100, Jan Kara wrote: > > > Hello, > > > > > > I've spent today tracking down the syzkaller report of a WARN_ON hit in > > > iov_iter_pipe() [1]. The immediate problem is that syzkaller reproducer > > > (calling sendfile(2) from different threads at the same time a file to the > > > same file in rather evil way) results in splice code leaking pipe pages > > > (nrbufs doesn't return to 0 after read+write in the splice) and eventually > > > we run out of pipe pages and hit the warning in iov_iter_pipe(). The > > > problem is not specific to ext4, I can see in my tracing that when the > > > underlying filesystem is XFS, we can leak the pipe pages in the same way > > > (but for XFS somehow the problem doesn't happen as often). Rather the > > > problem seems to be in how iomap direct IO code, pipe iter code, and splice > > > code interact. > > > > > > So the problematic situation is when we do direct IO read into pipe pages > > > and the read hits EOF which is not on page boundary. Say the file has 4608 > > > (4096+512) bytes, block size == page size == 4096. What happens is that iomap > > > code maps the extent, gets that the extent size is 8192 (mapping ignores > > > > I wonder, would this work properly if the read side returns a 4608-byte > > mapping instead of an 8192-byte mapping? It doesn't make a lot of sense > > (to me, anyway) for a read mapping to go beyond EOF. > > The slight concern I have with this is that that would change e.g. the > behavior of IOMAP_REPORT. We could specialcase IOMAP_REPORT but then it > gets kind of ugly. And it seems kind of fuzzy when do we truncate the > extent with i_size and when not... Generally i_size is kind of a side-band > thing for block mapping operations so if we could leave it out of > ->iomap_begin I'd find that nicer. > > > i_size). Then we call iomap_dio_bio_actor(), which creates its private > > > iter, truncates it to 8192, and calls bio_iov_iter_get_pages(). That > > > eventually results in preparing two pipe buffers with length 4096 to accept > > > the read. Then read completes, in iomap_dio_complete() we truncate the return > > > value from 8192 (which was the real amount of IO we performed) to 4608. Now > > > this amount (4608) gets passed through splice code to > > > iter_file_splice_write(), we write out that amount, but then when cleaning > > > up pipe buffers, the last pipe buffer has still 3584 unused so we leave > > > the pipe buffer allocated and effectively leak it. > > > > > > Now I was also investigating why the old direct IO code doesn't leak pipe > > > buffers like this and the trick is done by the iov_iter_revert() call > > > generic_file_read_iter(). This results in setting iter position right to > > > the position where direct IO read reported it ended (4608) and truncating > > > pipe buffers after this point. So splice code then sees the second pipe > > > buffer has length only 512 which matches the amount it was asked to write > > > and so the pipe buffer gets freed after the write in > > > iter_file_splice_write(). > > > > > > The question is how to best fix this. The quick fix is to add > > > iov_iter_revert() call to iomap_dio_rw() so that in case of sync IO (we > > > always do only sync IO to pipes), we properly set iter position in case of > > > short read / write. But it looks somewhat hacky to me and this whole > > > interaction of iter and pipes looks fragile to me. > > > > > > Another option I can see is to truncate the iter to min(i_size-pos, length) in > > > iomap_dio_bio_actor() which *should* do the trick AFAICT. But I'm not sure > > > if it won't break something else. > > > > Do the truncation in ->iomap_begin on the read side, as I suggested above? > > Yes, that would be equivalent for this case. > > > > Any other ideas? > > > > > > As a side note the logic copying iter in iomap_dio_bio_actor() looks > > > suspicious. We copy 'dio->submit.iter' to 'iter' but then in the loop we call > > > iov_iter_advance() on dio->submit.iter. So if bio_iov_iter_get_pages() > > > didn't return enough pages and we loop again, 'iter' will have stale > > > contents and things go sideways from there? What am I missing? And why do > > > we do that strange copying of iter instead of using iov_iter_truncate() and > > > iov_iter_reexpand() on the 'dio->submit.iter' directly? > > > > I'm similarly puzzled; I would've thought that we'd need to advance the > > private @iter too. Or just truncate and reexpand the dio->submit.iter > > and not have the private one. > > > > With any luck hch will have some ideas? :/ > > Christoph seems to be busy with something else. So I'll just write patches, > run them through fstests and see if something blows up. Heheh. Ok, sounds good! --D > Honza > -- > Jan Kara > SUSE Labs, CR