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 81C86C433F5 for ; Mon, 10 Jan 2022 20:36:15 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id BB1DF6B0071; Mon, 10 Jan 2022 15:36:14 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id B61966B0073; Mon, 10 Jan 2022 15:36:14 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A27C36B0074; Mon, 10 Jan 2022 15:36:14 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0096.hostedemail.com [216.40.44.96]) by kanga.kvack.org (Postfix) with ESMTP id 943856B0071 for ; Mon, 10 Jan 2022 15:36:14 -0500 (EST) Received: from smtpin20.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 55317181D6CFA for ; Mon, 10 Jan 2022 20:36:14 +0000 (UTC) X-FDA: 79015534668.20.8087FD1 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by imf13.hostedemail.com (Postfix) with ESMTP id B17412000C for ; Mon, 10 Jan 2022 20:36:13 +0000 (UTC) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id 0120C1F383; Mon, 10 Jan 2022 20:36:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1641846972; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=3Ji3UrzeuLEBRz8IsMGdO80u9dc3Tjl28ly6eTV4N+w=; b=LZ546GuAzAwuV41zx/O5frHHg4NWK4LkU8yUY7alkQMcSpJqRSQMQSjk4W9f27kHfpYEOQ G108JO2Js+KFfCBsfDE+8YT7r8rSnLOI/XxMLwQs5JJndzV8TzfhPRO19Wy+MmWKYv9kOC 312Fbw6RoqiPCTbRRRovNsnOZQNGZTk= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1641846972; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=3Ji3UrzeuLEBRz8IsMGdO80u9dc3Tjl28ly6eTV4N+w=; b=g54I1lAoOjzoqBW6Enb9JnJhLNq9EFocKKjFJOpfQPeW3FQf04obvXYVZFVZRkMpq373+9 rjFXZiqon8aCwhCA== Received: from quack3.suse.cz (unknown [10.100.224.230]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 7B4BBA3B84; Mon, 10 Jan 2022 20:36:11 +0000 (UTC) Received: by quack3.suse.cz (Postfix, from userid 1000) id 3DDCEA05A2; Mon, 10 Jan 2022 21:36:11 +0100 (CET) Date: Mon, 10 Jan 2022 21:36:11 +0100 From: Jan Kara To: Matthew Wilcox Cc: Jan Kara , John Hubbard , linux-mm@kvack.org, Andrew Morton , Christoph Hellwig Subject: Re: [PATCH 14/17] gup: Convert for_each_compound_head() to gup_for_each_folio() Message-ID: <20220110203611.7s2lg4cyejj5l5ah@quack3.lan> References: <20220102215729.2943705-1-willy@infradead.org> <20220102215729.2943705-15-willy@infradead.org> <20c2d9d3-bbbe-2f11-f6bf-a0e3578c6a71@nvidia.com> <20220110152208.w3tj5hjnbwjd6n2l@quack3.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: B17412000C X-Stat-Signature: g77dg37yy9wtp993d3fes4kr6614xnpc Authentication-Results: imf13.hostedemail.com; dkim=pass header.d=suse.cz header.s=susede2_rsa header.b=LZ546GuA; dkim=pass header.d=suse.cz header.s=susede2_ed25519 header.b=g54I1lAo; spf=pass (imf13.hostedemail.com: domain of jack@suse.cz designates 195.135.220.29 as permitted sender) smtp.mailfrom=jack@suse.cz; dmarc=none X-HE-Tag: 1641846973-587866 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 Mon 10-01-22 15:52:51, Matthew Wilcox wrote: > On Mon, Jan 10, 2022 at 04:22:08PM +0100, Jan Kara wrote: > > On Sun 09-01-22 00:01:49, John Hubbard wrote: > > > On 1/8/22 20:39, Matthew Wilcox wrote: > > > > On Wed, Jan 05, 2022 at 12:17:46AM -0800, John Hubbard wrote: > > > > > > + if (!folio_test_dirty(folio)) { > > > > > > + folio_lock(folio); > > > > > > + folio_mark_dirty(folio); > > > > > > + folio_unlock(folio); > > > > > > > > > > At some point, maybe even here, I suspect that creating the folio > > > > > version of set_page_dirty_lock() would help. I'm sure you have > > > > > a better feel for whether it helps, after doing all of this conversion > > > > > work, but it just sort of jumped out at me as surprising to see it > > > > > in this form. > > > > > > > > I really hate set_page_dirty_lock(). It smacks of "there is a locking > > > > rule here which we're violating, so we'll just take the lock to fix it" > > > > without understanding why there's a locking problem here. > > > > > > > > As far as I can tell, originally, the intent was that you would lock > > > > the page before modifying any of the data in the page. ie you would > > > > do: > > > > > > > > gup() > > > > lock_page() > > > > addr = kmap_page() > > > > *addr = 1; > > > > kunmap_page() > > > > set_page_dirty() > > > > unlock_page() > > > > put_page() > > > > > > > > and that would prevent races between modifying the page and (starting) > > > > writeback, not to mention truncate() and various other operations. > > > > > > > > Clearly we can't do that for DMA-pinned pages. There's only one lock > > > > bit. But do we even need to take the lock if we have the page pinned? > > > > What are we protecting against? > > > > > > This is a fun question, because you're asking it at a point when the > > > overall problem remains unsolved. That is, the interaction between > > > file-backed pages and gup/pup is still completely broken. > > > > > > And I don't have an answer for you: it does seem like lock_page() is > > > completely pointless here. Looking back, there are some 25 callers of > > > unpin_user_pages_dirty_lock(), and during all those patch reviews, no > > > one noticed this point! > > > > I'd say it is underdocumented but not obviously pointless :) AFAIR (and > > Christoph or Andrew may well correct me) the page lock in > > set_page_dirty_lock() is there to protect metadata associated with the page > > through page->private. Otherwise truncate could free these (e.g. > > block_invalidatepage()) while ->set_page_dirty() callback (e.g. > > __set_page_dirty_buffers()) works on this metadata. > > Yes, but ... we have an inconsistency between DMA writes to the page and > CPU writes to the page. > > fd = open(file) > write(fd, 1024 * 1024) > mmap(NULL, 1024 * 1024, PROT_RW, MAP_SHARED, fd, 0) > register-memory-with-RDMA > ftruncate(fd, 0); // page is removed from page cache > ftruncate(fd, 1024 * 1024) > > Now if we do a store from the CPU, we instantiate a new page in the > page cache and the store will be written back to the file. If we do > an RDMA-write, the write goes to the old page and will be lost. Indeed, > it's no longer visible to the CPU (but is visible to other RDMA reads!) > > Which is fine if the program did it itself because it's doing something > clearly bonkers, but another program might be the one doing the > two truncate() steps, and this would surprise an innocent program. > > I still favour blocking the truncate-down (or holepunch) until there > are no pinned pages in the inode. But I know this is a change in > behaviour since for some reason, truncate() gets to override mmap(). I agree although this is unrelated to the page lock discussion above. In principle we can consider such change (after all we chose this solution for DAX) but it has some consequences - e.g. that disk space cannot be reclaimed when someone has pagecache pages pinned (which may be unexpected from sysadmin POV) or that we have to be careful or eager application doing DIO (once it is converted to pinning) can block truncate indefinitely. Honza -- Jan Kara SUSE Labs, CR