From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752997AbbKIHmZ (ORCPT ); Mon, 9 Nov 2015 02:42:25 -0500 Received: from mail-pa0-f42.google.com ([209.85.220.42]:35146 "EHLO mail-pa0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752956AbbKIHmU (ORCPT ); Mon, 9 Nov 2015 02:42:20 -0500 Date: Sun, 8 Nov 2015 23:42:16 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Mike Kravetz cc: Hugh Dickins , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Dave Hansen , Naoya Horiguchi , Davidlohr Bueso Subject: Re: [PATCH] mm/hugetlb: Unmap pages if page fault raced with hole punch In-Reply-To: <5633D984.7080307@oracle.com> Message-ID: References: <1446158038-25815-1-git-send-email-mike.kravetz@oracle.com> <56339EBA.4070508@oracle.com> <5633D984.7080307@oracle.com> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 30 Oct 2015, Mike Kravetz wrote: > > The 'next = start' code is actually from the original truncate_hugepages > routine. This functionality was combined with that needed for hole punch > to create remove_inode_hugepages(). > > The following code was in truncate_hugepages: > > next = start; > while (1) { > if (!pagevec_lookup(&pvec, mapping, next, PAGEVEC_SIZE)) { > if (next == start) > break; > next = start; > continue; > } > > > So, in the truncate case pages starting at 'start' are deleted until > pagevec_lookup fails. Then, we call pagevec_lookup() again. If no > pages are found we are done. Else, we repeat the whole process. > > Does anyone recall the reason for going back and looking for pages at > index'es already deleted? Git doesn't help as that was part of initial > commit. My thought is that truncate can race with page faults. The > truncate code sets inode offset before unmapping and deleting pages. > So, faults after the new offset is set should fail. But, I suppose a > fault could race with setting offset and deleting of pages. Does this > sound right? Or, is there some other reason I am missing? I believe your thinking is correct. But remember that truncate_inode_pages_range() is shared by almost all filesystems, and different filesystems have different internal locking conventions, and different propensities to such a race: it's trying to cover for all of them. Typically, writing is well serialized (by i_mutex) against truncation, but faulting (like reading) sails through without enough of a lock. We resort to i_size checks to avoid the worst of it, but there's often a corner or two in which those checks are not quite good enough - it's easy to check i_size at the beginning, but it needs to be checked again at the end too, and what's been done undone - can be awkward. I hope that in the case of hugetlbfs, since you already have the additional fault_mutex to handle races between faults and punching, it should be possible to get away without that "pincer" restarting. Hugh > > I would like to continue having remove_inode_hugepages handle both the > truncate and hole punch case. So, what to make sure the code correctly > handles both cases. > > -- > Mike Kravetz