From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with ESMTP id E71BD62009A for ; Thu, 6 May 2010 11:52:03 -0400 (EDT) Date: Thu, 6 May 2010 16:51:43 +0100 From: Mel Gorman Subject: Re: [PATCH 1/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information Message-ID: <20100506155143.GD8704@csn.ul.ie> References: <1273159987-10167-1-git-send-email-mel@csn.ul.ie> <1273159987-10167-2-git-send-email-mel@csn.ul.ie> <4BE2E3F9.9090708@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <4BE2E3F9.9090708@redhat.com> Sender: owner-linux-mm@kvack.org To: Rik van Riel Cc: Andrew Morton , Linux-MM , LKML , Minchan Kim , KAMEZAWA Hiroyuki , Christoph Lameter , Andrea Arcangeli , Linus Torvalds , Peter Zijlstra List-ID: On Thu, May 06, 2010 at 11:44:57AM -0400, Rik van Riel wrote: > On 05/06/2010 11:33 AM, Mel Gorman wrote: > >> @@ -1368,16 +1424,25 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *, >> * are holding mmap_sem. Users without mmap_sem are required to >> * take a reference count to prevent the anon_vma disappearing >> */ >> - anon_vma = page_anon_vma(page); >> + anon_vma = page_anon_vma_lock_root(page); >> if (!anon_vma) >> return ret; >> - spin_lock(&anon_vma->lock); >> list_for_each_entry(avc,&anon_vma->head, same_anon_vma) { > > One conceptual problem here. By taking the oldest anon_vma, > instead of the anon_vma of the page, we may end up searching > way too many processes. > > Eg. if the page is the page of a child process in a forking > server workload, the above code will end up searching the > parent and all of the siblings - even for a private page, in > the child process's private anon_vma. > > For an Apache or Oracle system with 1000 clients (and child > processes), that could be quite a drag - searching 1000 times > as many processes as we should. > That does indeed suck. If we were always locking the root anon_vma, we'd get away with it but that would involve introducing RCU into the munmap/mmap/etc path. Is there any way around this problem or will migration just have to take it on the chin until anon_vma is reference counted and we can cheaply lock the root anon_vma? -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- 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/ . Don't email: email@kvack.org