Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932351AbaLBPGf (ORCPT ); Tue, 2 Dec 2014 10:06:35 -0500 Received: from gum.cmpxchg.org ([85.214.110.215]:51283 "EHLO gum.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753535AbaLBPGd (ORCPT ); Tue, 2 Dec 2014 10:06:33 -0500 Date: Tue, 2 Dec 2014 10:06:23 -0500 From: Johannes Weiner To: Jan Kara Cc: Andrew Morton , Tejun Heo , Hugh Dickins , Michel Lespinasse , 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: <20141202150623.GA8401@phnom.home.cmpxchg.org> References: <1417474682-29326-1-git-send-email-hannes@cmpxchg.org> <20141202091212.GB9092@quack.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141202091212.GB9092@quack.suse.cz> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 02, 2014 at 10:12:12AM +0100, Jan Kara wrote: > On Mon 01-12-14 17:58:00, 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: > > --- > > include/linux/writeback.h | 1 - > > mm/memory.c | 26 ++++++++++++++++---------- > > mm/page-writeback.c | 43 ++++++++++++------------------------------- > > 3 files changed, 28 insertions(+), 42 deletions(-) > > > > diff --git a/include/linux/writeback.h b/include/linux/writeback.h > > index a219be961c0a..00048339c23e 100644 > > --- a/include/linux/writeback.h > > +++ b/include/linux/writeback.h > > @@ -177,7 +177,6 @@ int write_cache_pages(struct address_space *mapping, > > struct writeback_control *wbc, writepage_t writepage, > > void *data); > > int do_writepages(struct address_space *mapping, struct writeback_control *wbc); > > -void set_page_dirty_balance(struct page *page); > > void writeback_set_ratelimit(void); > > void tag_pages_for_writeback(struct address_space *mapping, > > pgoff_t start, pgoff_t end); > > diff --git a/mm/memory.c b/mm/memory.c > > index 3e503831e042..73220eb6e9e3 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -2150,17 +2150,23 @@ reuse: > > if (!dirty_page) > > return ret; > > > > - /* > > - * Yes, Virginia, this is actually required to prevent a race > > - * with clear_page_dirty_for_io() from clearing the page dirty > > - * bit after it clear all dirty ptes, but before a racing > > - * do_wp_page installs a dirty pte. > > - * > > - * do_shared_fault is protected similarly. > > - */ > > if (!page_mkwrite) { > > - wait_on_page_locked(dirty_page); > > - set_page_dirty_balance(dirty_page); > > + struct address_space *mapping; > > + int dirtied; > > + > > + lock_page(dirty_page); > > + dirtied = set_page_dirty(dirty_page); > > + mapping = dirty_page->mapping; > > + unlock_page(dirty_page); > > + > > + if (dirtied && mapping) { > > + /* > > + * Some device drivers do not set page.mapping > > + * but still dirty their pages > > + */ > The comment doesn't make sense to me here. Is it meant to explain why we > check 'mapping' in the above condition? I always thought truncate is the > main reason. Yes, I just copied it from the page_mkwrite case a few lines down, and there is another copy of it in do_shared_fault(). Truncate is also a possibility during a race, of course, but even without it we expect that the mapping can be NULL for certain device drivers. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/