Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754945AbdDJPvg (ORCPT ); Mon, 10 Apr 2017 11:51:36 -0400 Received: from mx2.suse.de ([195.135.220.15]:40511 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754431AbdDJPvd (ORCPT ); Mon, 10 Apr 2017 11:51:33 -0400 Date: Mon, 10 Apr 2017 17:51:29 +0200 From: Jan Kara To: alexander.levin@verizon.com Cc: Jan Kara , Johannes Weiner , Andrew Morton , Tejun Heo , Hugh Dickins , Michel Lespinasse , "Kirill A. Shutemov" , "linux-mm@kvack.org" , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [patch 1/3] mm: protect set_page_dirty() from ongoing truncation Message-ID: <20170410155129.GK3224@quack2.suse.cz> References: <1417791166-32226-1-git-send-email-hannes@cmpxchg.org> <20170410022230.xe5sukvflvoh4ula@sasha-lappy> <20170410120638.GD3224@quack2.suse.cz> <20170410150755.kd2gjqyfmvschtxd@sasha-lappy> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170410150755.kd2gjqyfmvschtxd@sasha-lappy> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3585 Lines: 87 On Mon 10-04-17 15:07:58, alexander.levin@verizon.com wrote: > On Mon, Apr 10, 2017 at 02:06:38PM +0200, Jan Kara wrote: > > On Mon 10-04-17 02:22:33, alexander.levin@verizon.com wrote: > > > On Fri, Dec 05, 2014 at 09:52:44AM -0500, Johannes Weiner wrote: > > > > Tejun, while reviewing the code, spotted the following race condition > > > > between the dirtying and truncation of a page: > > > > > > > > __set_page_dirty_nobuffers() __delete_from_page_cache() > > > > if (TestSetPageDirty(page)) > > > > page->mapping = NULL > > > > if (PageDirty()) > > > > dec_zone_page_state(page, NR_FILE_DIRTY); > > > > dec_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE); > > > > if (page->mapping) > > > > account_page_dirtied(page) > > > > __inc_zone_page_state(page, NR_FILE_DIRTY); > > > > __inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE); > > > > > > > > which results in an imbalance of NR_FILE_DIRTY and BDI_RECLAIMABLE. > > > > > > > > Dirtiers usually lock out truncation, either by holding the page lock > > > > directly, or in case of zap_pte_range(), by pinning the mapcount with > > > > the page table lock held. The notable exception to this rule, though, > > > > is do_wp_page(), for which this race exists. However, do_wp_page() > > > > already waits for a locked page to unlock before setting the dirty > > > > bit, in order to prevent a race where clear_page_dirty() misses the > > > > page bit in the presence of dirty ptes. Upgrade that wait to a fully > > > > locked set_page_dirty() to also cover the situation explained above. > > > > > > > > Afterwards, the code in set_page_dirty() dealing with a truncation > > > > race is no longer needed. Remove it. > > > > > > > > Reported-by: Tejun Heo > > > > Signed-off-by: Johannes Weiner > > > > Cc: > > > > Acked-by: Kirill A. Shutemov > > > > > > Hi Johannes, > > > > > > I'm seeing the following while fuzzing with trinity on linux-next (I've changed > > > the WARN to a VM_BUG_ON_PAGE for some extra page info). > > > > But this looks more like a bug in 9p which allows v9fs_write_end() to dirty > > a !Uptodate page? > > I thought that 77469c3f5 ("9p: saner ->write_end() on failing copy into > non-uptodate page") prevented from that happening, but that's actually the > change that's causing it (I ended up misreading it last night). > > Will fix it as follows: Yep, this looks good to me, although I'd find it more future-proof if we had that SetPageUptodate() additionally guarded a by len == PAGE_SIZE check. Honza > > diff --git a/fs/9p/vfs_addr.c b/fs/9p/vfs_addr.c > index adaf6f6..be84c0c 100644 > --- a/fs/9p/vfs_addr.c > +++ b/fs/9p/vfs_addr.c > @@ -310,9 +310,13 @@ static int v9fs_write_end(struct file *filp, struct address_space *mapping, > > p9_debug(P9_DEBUG_VFS, "filp %p, mapping %p\n", filp, mapping); > > - if (unlikely(copied < len && !PageUptodate(page))) { > - copied = 0; > - goto out; > + if (!PageUptodate(page)) { > + if (unlikely(copied < len)) { > + copied = 0; > + goto out; > + } else { > + SetPageUptodate(page); > + } > } > /* > * No need to use i_size_read() here, the i_size > > -- > > Thanks, > Sasha -- Jan Kara SUSE Labs, CR