From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with SMTP id CD03D6007B8 for ; Mon, 3 May 2010 14:39:26 -0400 (EDT) Message-ID: <4BDF1840.7020601@redhat.com> Date: Mon, 03 May 2010 14:38:56 -0400 From: Rik van Riel MIME-Version: 1.0 Subject: Re: [PATCH 1/2] mm: Take all anon_vma locks in anon_vma_lock References: <20100503121743.653e5ecc@annuminas.surriel.com> <20100503121847.7997d280@annuminas.surriel.com> <4BDEFF9E.6080508@redhat.com> <4BDF0ECC.5080902@redhat.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Linus Torvalds Cc: akpm@linux-foundation.org, Mel Gorman , Linux-MM , LKML , Minchan Kim , KAMEZAWA Hiroyuki , Andrea Arcangeli , Christoph Lameter List-ID: On 05/03/2010 02:19 PM, Linus Torvalds wrote: > On Mon, 3 May 2010, Rik van Riel wrote: >> >> One problem is that we cannot find the VMAs (multiple) from >> the page, except by walking the anon_vma_chain.same_anon_vma >> list. At the very least, that list requires locking, done >> by the anon_vma.lock. > > But that's exactly what we do in rmap_walk() anyway. Mel's original patch adds trylock & retry all code to rmap_walk and a few other places: http://lkml.org/lkml/2010/4/26/321 I submitted my patch 1/2 as an alternative, because these repeated trylocks are pretty complex and easy to accidentally break when changes to other VM code are made. >> A forkbomb could definately end up getting slowed down by >> this patch. Is there any real workload out there that just >> forks deeper and deeper from the parent process, without >> calling exec() after a generation or two? > > Heh. AIM7. Wasn't that why we merged the multiple anon_vma's in the first > place? AIM7, like sendmail, apache or postgresql, is only 2 deep. >>> So again, my gut feel is that if the lock just were in the vma itself, >>> then the "normal" users would have just one natural lock, while the >>> special case users (rmap_walk_anon) would have to lock each vma it >>> traverses. That would seem to be the more natural way to lock things. >> >> However ... there's still the issue of page_lock_anon_vma >> in try_to_unmap_anon. > > Do we care? > > We've not locked them all there, and we've historically not cares about > the rmap list being "perfect", have we? Well, try_to_unmap_anon walks just one page, and has the anon_vma for that page locked. Having said that, for pageout we do indeed not care about getting it perfect. > So I _think_ it's just the migration case (and apparently potentially the > hugepage case) that wants _exact_ information. Which is why I suggest the > onus of the extra locking should be on _them_, not on the regular code. It's a matter of cost vs complexity. IMHO the locking changes in the lowest overhead patches (Mel's) are quite complex and could end up being hard to maintain in the future. I wanted to introduce something a little simpler, with hopefully minimal overhead. But hey, that's just my opinion - what matters is that the bug gets fixed somehow. If you prefer the more complex but slightly lower overhead patches from Mel, that's fine too. -- All rights reversed -- 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