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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E1366C6FA89 for ; Wed, 14 Sep 2022 03:51:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229850AbiINDvn (ORCPT ); Tue, 13 Sep 2022 23:51:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38304 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229824AbiINDvl (ORCPT ); Tue, 13 Sep 2022 23:51:41 -0400 Received: from zeniv.linux.org.uk (zeniv.linux.org.uk [IPv6:2a03:a000:7:0:5054:ff:fe1c:15ff]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 430606F26F; Tue, 13 Sep 2022 20:51:38 -0700 (PDT) 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: Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org 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);