From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx183.postini.com [74.125.245.183]) by kanga.kvack.org (Postfix) with SMTP id 243446B0044 for ; Thu, 15 Mar 2012 19:27:14 -0400 (EDT) Date: Thu, 15 Mar 2012 16:27:11 -0700 From: Andrew Morton Subject: Re: [PATCH] mm: thp: fix pmd_bad() triggering in code paths holding mmap_sem read mode Message-Id: <20120315162711.0870c27b.akpm@linux-foundation.org> In-Reply-To: <20120315231556.GA24602@redhat.com> References: <1331822671-21508-1-git-send-email-aarcange@redhat.com> <20120315154504.0fe15f95.akpm@linux-foundation.org> <20120315231556.GA24602@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrea Arcangeli Cc: linux-mm@kvack.org, Johannes Weiner , Mel Gorman , Hugh Dickins , Larry Woodman , Rik van Riel , Ulrich Obergfell , Naoya Horiguchi On Fri, 16 Mar 2012 00:15:56 +0100 Andrea Arcangeli wrote: > On Thu, Mar 15, 2012 at 03:45:04PM -0700, Andrew Morton wrote: > > Or do we still need pdm_trans_unstable() checking in > > mem_cgroup_count_precharge_pte_range() and > > mem_cgroup_move_charge_pte_range()? > > I think we need a pmd_trans_unstable check before the > pte_offset_map_lock in both places. Otherwise with only the mmap_sem > hold for reading, the pmd may have been transhuge, > mem_cgroup_move_charge_pte_range could be called, and then > MADV_DONTNEED would transform the pmd to none from another thread just > before pmd_trans_huge_lock runs, and we would end up doing > pmd_offset_map_lock on a none pmd (or a transhuge pmd if it becomes > huge again before we get there). page_table_lock doesn't prevent the race? pmd_trans_huge_lock() rechecks after taking that lock... > Only if pmd_trans_unstable is false, the pmd can't change from under > us, so we can proceed safely with the pte level walk (and it just need > to be checked with a compiler barrier, as the real pmd changes freely > from under us). > > pmd_trans_unstable will never actually trigger unless we're hitting > the race, if the pmd was none when we started the walk we'd abort at > the higher level (method not called), if the pmd was transhuge we'd > stop at the pmd_trans_huge_lock() == 1 branch. So the only way to run > pmd_trans_unstable is when the result is undefined, i.e. the pmd was > not none initially but it become none or transhuge or none again at > some point, so we can just simply consider it still none and skip for > the undefined case. Naoya, could you please take a look into this? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org