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 DD0CB6B029E for ; Wed, 5 May 2010 10:56:44 -0400 (EDT) Date: Wed, 5 May 2010 15:56:20 +0100 From: Mel Gorman Subject: Re: [PATCH 1/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information Message-ID: <20100505145620.GP20979@csn.ul.ie> References: <1273065281-13334-1-git-send-email-mel@csn.ul.ie> <1273065281-13334-2-git-send-email-mel@csn.ul.ie> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org To: Linus Torvalds Cc: Andrew Morton , Linux-MM , LKML , Minchan Kim , KAMEZAWA Hiroyuki , Christoph Lameter , Andrea Arcangeli , Rik van Riel List-ID: On Wed, May 05, 2010 at 07:34:37AM -0700, Linus Torvalds wrote: > > > On Wed, 5 May 2010, Mel Gorman wrote: > > > > With the recent anon_vma changes, there can be more than one anon_vma->lock > > to take in a anon_vma_chain but a second lock cannot be spinned upon in case > > of deadlock. The rmap walker tries to take locks of different anon_vma's > > but if the attempt fails, locks are released and the operation is restarted. > > Btw, is this really needed? > I could not convince myself that it wasn't. lockdep throws a fit if you try but it can be taught about the situation if necessary. > Nobody else takes two anon_vma locks at the same time, so in order to > avoid ABBA deadlocks all we need to guarantee is that rmap_walk_ksm() and > rmap_walk_anon() always lock the anon_vma's in the same order. > rmap_walk() appears to be the only one that takes multiple locks but it itself is not serialised. If there are more than one process calling rmap_walk() on different processes sharing the same VMAs, is there a guarantee they walk it in the same order? I didn't think so at the time the patch because the anon_vma the walk starts from is based on the page being migrated rather than any idea of starting from a parent or primary anon_vma. > And they do, as far as I can tell. How could we ever get a deadlock when > we have both cases doing the locking by walking the same_anon_vma list? > If we always started the list walk in the same place then it'd be fine but if they start in different places, it could deadlock. > list_for_each_entry(avc, &anon_vma->head, same_anon_vma) { > > So I think the "retry" logic looks unnecessary, and actually opens us up > to a possible livelock bug (imagine a long chain, and heavy page fault > activity elsewhere that ends up locking some anon_vma in the chain, and > just the right behavior that gets us into a lockstep situation), I imagined it and I'm not super-happy about it. It's one of the reasons Rik called it "fragile". > rather than fixing an ABBA deadlock. > > Now, if it's true that somebody else _does_ do nested anon_vma locking, > I'm obviously wrong. But I don't see such usage. > > Comments? > Just what I have above. I couldn't convince myself that two callers to rmap_walk from pages based on different VMAs on the same_anon_vma list would always started in the same place. -- 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