Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755720AbaBROQf (ORCPT ); Tue, 18 Feb 2014 09:16:35 -0500 Received: from mga02.intel.com ([134.134.136.20]:19283 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755128AbaBROQd convert rfc822-to-8bit (ORCPT ); Tue, 18 Feb 2014 09:16:33 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.97,502,1389772800"; d="scan'208";a="485203869" From: "Wilcox, Matthew R" To: Rik van Riel , Linus Torvalds , "Kirill A. Shutemov" CC: Andrew Morton , Mel Gorman , Andi Kleen , Dave Hansen , Alexander Viro , Dave Chinner , linux-mm , linux-fsdevel , Linux Kernel Mailing List Subject: RE: [RFC, PATCHv2 0/2] mm: map few pages around fault address if they are in page cache Thread-Topic: [RFC, PATCHv2 0/2] mm: map few pages around fault address if they are in page cache Thread-Index: AQHPLA+LtMkhTgsne06MzzyLvYMEt5q6U3MAgAE1CAD//3vEjg== Date: Tue, 18 Feb 2014 14:15:59 +0000 Message-ID: <100D68C7BA14664A8938383216E40DE04062DEA1@FMSMSX114.amr.corp.intel.com> References: <1392662333-25470-1-git-send-email-kirill.shutemov@linux.intel.com> ,<53035FE2.4080300@redhat.com> In-Reply-To: <53035FE2.4080300@redhat.com> Accept-Language: en-CA, en-US Content-Language: en-CA X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [10.19.9.46] Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8BIT MIME-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org We don't really need to lock all the pages being returned to protect against truncate. We only need to lock the one at the highest index, and check i_size while that lock is held since truncate_inode_pages_range() will block on any page that is locked. We're still vulnerable to holepunches, but there's no locking currently between holepunches and truncate, so we're no worse off now. ________________________________________ From: Rik van Riel [riel@redhat.com] Sent: February 18, 2014 5:28 AM To: Linus Torvalds; Kirill A. Shutemov Cc: Andrew Morton; Mel Gorman; Andi Kleen; Wilcox, Matthew R; Dave Hansen; Alexander Viro; Dave Chinner; linux-mm; linux-fsdevel; Linux Kernel Mailing List Subject: Re: [RFC, PATCHv2 0/2] mm: map few pages around fault address if they are in page cache On 02/17/2014 02:01 PM, Linus Torvalds wrote: > - increment the page _mapcount (iow, do "page_add_file_rmap()" > early). This guarantees that any *subsequent* unmap activity on this > page will walk the file mapping lists, and become serialized by the > page table lock we hold. > > - mb_after_atomic_inc() (this is generally free) > > - test that the page is still unlocked and uptodate, and the page > mapping still points to our page. > > - if that is true, we're all good, we can use the page, otherwise we > decrement the mapcount (page_remove_rmap()) and skip the page. > > Hmm? Doing something like this means that we would never lock the > pages we prefault, and you can go back to your gang lookup rather than > that "one page at a time". And the race case is basically never going > to trigger. > > Comments? What would the direct io code do when it runs into a page with elevated mapcount, but for which a mapping cannot be found yet? Looking at the code, it looks like the above scheme could cause some trouble with invalidate_inode_pages2_range(), which has the following sequence: if (page_mapped(page)) { ... unmap page } BUG_ON(page_mapped(page)); In other words, it looks like incrementing _mapcount first could lead to an oops in the truncate and direct IO code. The page lock is used to prevent such races. *sigh* -- All rights reversed -- 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/