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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5DF3BC6FA82 for ; Wed, 14 Sep 2022 03:51:44 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 974E18D0001; Tue, 13 Sep 2022 23:51:43 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 924E66B0073; Tue, 13 Sep 2022 23:51:43 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 813458D0001; Tue, 13 Sep 2022 23:51:43 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id 6E3496B0071 for ; Tue, 13 Sep 2022 23:51:43 -0400 (EDT) Received: from smtpin03.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 3E60540127 for ; Wed, 14 Sep 2022 03:51:43 +0000 (UTC) X-FDA: 79909316886.03.BC83879 Received: from zeniv.linux.org.uk (zeniv.linux.org.uk [62.89.141.173]) by imf20.hostedemail.com (Postfix) with ESMTP id A82DA1C00BF for ; Wed, 14 Sep 2022 03:51:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=linux.org.uk; s=zeniv-20220401; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=TsvPRmshdxnvSY7tVAecRy6+/ce+5hAWcS0J6w4JVyE=; b=utEF1SOCThG3gcFxitxIjq3WGs IHfq+Z1KW6tA+eL0Sb+cwipStWaYLjhRhDVZ8GZwNeZmjvBd05BRjtyauZaHH7ZZsrmJ5jQ9m7NYH t5YnppvGuDY68g9HYjVC2w5Z2W2ZRP72TY/rLYf5gK5zcB6xl2TQzc8waMQW+L4/KmLbLr+tX1SHf wva/bWAZqkuKFcKia9aZdCn5gJ0/PD4X1KzRVwCHXE2T9wrl7lomyCDzaAAQRhl9Ob3eHi4ibY6gS 2A/E8y+stzUFsaCfnKU1fUsoMPrEmJ9V+q/Xc7gYpbpZ/qBuLSMLG/JKoH2axtiXJ3qbsfdU1SaT+ ODK9JccA==; Received: from viro by zeniv.linux.org.uk with local (Exim 4.96 #2 (Red Hat Linux)) id 1oYJQj-00G2aP-2o; Wed, 14 Sep 2022 03:51:17 +0000 Date: Wed, 14 Sep 2022 04:51:17 +0100 From: Al Viro To: Christoph Hellwig Cc: Jan Kara , John Hubbard , Andrew Morton , Jens Axboe , Miklos Szeredi , "Darrick J . Wong" , Trond Myklebust , Anna Schumaker , David Hildenbrand , Logan Gunthorpe , linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, LKML Subject: Re: [PATCH v2 4/7] iov_iter: new iov_iter_pin_pages*() routines Message-ID: References: <20220831041843.973026-1-jhubbard@nvidia.com> <20220831041843.973026-5-jhubbard@nvidia.com> <103fe662-3dc8-35cb-1a68-dda8af95c518@nvidia.com> <20220906102106.q23ovgyjyrsnbhkp@quack3> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1663127502; h=from:from:sender:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=TsvPRmshdxnvSY7tVAecRy6+/ce+5hAWcS0J6w4JVyE=; b=662UqXmMu/IU4Q+savfju0mJrCUcaxy6mCPbC3K2I/mSjQ4OP3fIckKCNoocMPFvtpIGHz nLbTyFK2hJetw0mqqBuThVXTrspQQXxi+qXgvbPws9A+F7773UJkXOJ9dj5GMlFv5weEFj /M99ZYIBNk/WLewGMk1qZzagM9rhiG0= ARC-Authentication-Results: i=1; imf20.hostedemail.com; dkim=pass header.d=linux.org.uk header.s=zeniv-20220401 header.b=utEF1SOC; dmarc=pass (policy=none) header.from=zeniv.linux.org.uk; spf=none (imf20.hostedemail.com: domain of viro@ftp.linux.org.uk has no SPF policy when checking 62.89.141.173) smtp.mailfrom=viro@ftp.linux.org.uk ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1663127502; a=rsa-sha256; cv=none; b=twlqmsSm0/H6HZ7vp3uyOkNQZlgqPiuWeLSc9SRrJdUxk3JpmdgPluI4oneM86JgevgIj4 slxPPAsRvXfLI5NVJTwM5O48BgvLNSpcY6hB9bbGHdKaZLnkkXyhDAcf013+hIOhiqcnw8 w9ywenyycy+va95vsHM8C7X/D/IhEVA= Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=linux.org.uk header.s=zeniv-20220401 header.b=utEF1SOC; dmarc=pass (policy=none) header.from=zeniv.linux.org.uk; spf=none (imf20.hostedemail.com: domain of viro@ftp.linux.org.uk has no SPF policy when checking 62.89.141.173) smtp.mailfrom=viro@ftp.linux.org.uk X-Stat-Signature: w5154r4huz9hdto7oh3bssidix97yeep X-Rspamd-Queue-Id: A82DA1C00BF X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1663127502-410352 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Sep 07, 2022 at 01:45:26AM -0700, Christoph Hellwig wrote: > On Tue, Sep 06, 2022 at 12:21:06PM +0200, Jan Kara wrote: > > > For FOLL_PIN callers, never pin bvec and kvec pages: For file systems > > > not acquiring a reference is obviously safe, and the other callers will > > > need an audit, but I can't think of why it woul ever be unsafe. > > > > Are you sure about "For file systems not acquiring a reference is obviously > > safe"? I can see places e.g. in orangefs, afs, etc. which create bvec iters > > from pagecache pages. And then we have iter_file_splice_write() which > > creates bvec from pipe pages (which can also be pagecache pages if > > vmsplice() is used). So perhaps there are no lifetime issues even without > > acquiring a reference (but looking at the code I would not say it is > > obvious) but I definitely don't see how it would be safe to not get a pin > > to signal to filesystem backing the pagecache page that there is DMA > > happening to/from the page. > > I mean in the context of iov_iter_get_pages callers, that is direct > I/O. Direct callers of iov_iter_bvec which then pass that iov to > ->read_iter / ->write_iter will need to hold references (those are > the references that the callers of iov_iter_get_pages rely on!). Unless I'm misreading Jan, the question is whether they should get or pin. AFAICS, anyone who passes the sucker to ->read_iter() (or ->recvmsg(), or does direct copy_to_iter()/zero_iter(), etc.) is falling under ================================================================================= CASE 5: Pinning in order to write to the data within the page ------------------------------------------------------------- Even though neither DMA nor Direct IO is involved, just a simple case of "pin, write to a page's data, unpin" can cause a problem. Case 5 may be considered a superset of Case 1, plus Case 2, plus anything that invokes that pattern. In other words, if the code is neither Case 1 nor Case 2, it may still require FOLL_PIN, for patterns like this: Correct (uses FOLL_PIN calls): pin_user_pages() write to the data within the pages unpin_user_pages() INCORRECT (uses FOLL_GET calls): get_user_pages() write to the data within the pages put_page() ================================================================================= Regarding iter_file_splice_write() case, do we need to pin pages when we are not going to modify the data in those? The same goes for afs, AFAICS; I started to type "... and everything that passes WRITE to iov_iter_bvec()", but... drivers/vhost/vringh.c:1165: iov_iter_bvec(&iter, READ, iov, ret, translated); drivers/vhost/vringh.c:1198: iov_iter_bvec(&iter, WRITE, iov, ret, translated); is backwards - READ is for data destinations, comes with copy_to_iter(); WRITE is for data sources and it comes with copy_from_iter()... I'm really tempted to slap if (WARN_ON(i->data_source)) return 0; into copy_to_iter() et.al., along with its opposite for copy_from_iter(). And see who comes screaming... Things like if (unlikely(iov_iter_is_pipe(i) || iov_iter_is_discard(i))) { WARN_ON(1); return 0; } in e.g. csum_and_copy_from_iter() would be replaced by that, and become easier to understand... These two are also getting it wrong, BTW: drivers/target/target_core_file.c:340: iov_iter_bvec(&iter, READ, bvec, sgl_nents, len); drivers/target/target_core_file.c:476: iov_iter_bvec(&iter, READ, bvec, nolb, len);