From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx197.postini.com [74.125.245.197]) by kanga.kvack.org (Postfix) with SMTP id 48CCE6B0072 for ; Thu, 25 Oct 2012 17:48:39 -0400 (EDT) Received: by mail-ie0-f169.google.com with SMTP id 10so3695746ied.14 for ; Thu, 25 Oct 2012 14:48:38 -0700 (PDT) Date: Thu, 25 Oct 2012 14:48:39 -0700 (PDT) From: Hugh Dickins Subject: Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2] In-Reply-To: <20121025205213.GB4771@cmpxchg.org> Message-ID: References: <20121025023738.GA27001@redhat.com> <20121025205213.GB4771@cmpxchg.org> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner Cc: Dave Jones , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Thu, 25 Oct 2012, Johannes Weiner wrote: > On Wed, Oct 24, 2012 at 09:36:27PM -0700, Hugh Dickins wrote: > > On Wed, 24 Oct 2012, Dave Jones wrote: > > > > > Machine under significant load (4gb memory used, swap usage fluctuating) > > > triggered this... > > > > > > WARNING: at mm/shmem.c:1151 shmem_getpage_gfp+0xa5c/0xa70() > > > Pid: 29795, comm: trinity-child4 Not tainted 3.7.0-rc2+ #49 > > > > > > 1148 error = shmem_add_to_page_cache(page, mapping, index, > > > 1149 gfp, swp_to_radix_entry(swap)); > > > 1150 /* We already confirmed swap, and make no allocation */ > > > 1151 VM_BUG_ON(error); > > > 1152 } > > > > That's very surprising. Easy enough to handle an error there, but > > of course I made it a VM_BUG_ON because it violates my assumptions: > > I rather need to understand how this can be, and I've no idea. > > Could it be concurrent truncation clearing out the entry between > shmem_confirm_swap() and shmem_add_to_page_cache()? I don't see > anything preventing that. > > The empty slot would not match the expected swap entry this call > passes in and the returned error would be -ENOENT. Excellent notion, many thanks Hannes, I believe you've got it. I've hit that truncation problem in swapoff (and commented on it in shmem_unuse_inode), but never hit it or considered it here. I think of the page lock as holding it stable, but truncation's free_swap_and_cache only does a trylock on the swapcache page, so we're not secured against that possibility. So I'd like to change it to VM_BUG_ON(error && error != -ENOENT), but there's a little tidying up to do in the -ENOENT case, which needs more thought. A delete_from_swap_cache(page) - though we can be lazy and leave that to reclaim for such a rare occurrence - and probably a mem_cgroup uncharge; but the memcg hooks are always the hardest to get right, I'll have think about that one carefully. Hugh -- 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