From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with ESMTP id 55BD46B00E9 for ; Wed, 12 Jan 2011 04:26:24 -0500 (EST) Date: Wed, 12 Jan 2011 09:25:59 +0000 From: Mel Gorman Subject: Re: mmotm hangs on compaction lock_page Message-ID: <20110112092558.GG11932@csn.ul.ie> References: <20110107145259.GK29257@csn.ul.ie> <20110107175705.GL29257@csn.ul.ie> <20110110172609.GA11932@csn.ul.ie> <20110111114521.GD11932@csn.ul.ie> <20110111124551.f8d0522c.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20110111124551.f8d0522c.akpm@linux-foundation.org> Sender: owner-linux-mm@kvack.org To: Andrew Morton Cc: Hugh Dickins , linux-mm@kvack.org List-ID: On Tue, Jan 11, 2011 at 12:45:51PM -0800, Andrew Morton wrote: > On Tue, 11 Jan 2011 11:45:21 +0000 > Mel Gorman wrote: > > > --- a/mm/page_alloc.c > > +++ b/mm/page_alloc.c > > @@ -1809,12 +1809,15 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order, > > bool sync_migration) > > { > > struct page *page; > > + struct task_struct *p = current; > > > > if (!order || compaction_deferred(preferred_zone)) > > return NULL; > > > > + p->flags |= PF_MEMALLOC; > > *did_some_progress = try_to_compact_pages(zonelist, order, gfp_mask, > > nodemask, sync_migration); > > + p->flags &= ~PF_MEMALLOC; > > Thus accidentally wiping out PF_MEMALLOC if it was already set. > Subtle but we can't have reached here if PF_MEMALLOC was previous set. It gets caught by /* Avoid recursion of direct reclaim */ if (p->flags & PF_MEMALLOC) goto nopage; > It's risky, and general bad practice. The default operation here > should be to push the old value and to later restore it. > Arguably we would do this in case that check was ever removed but I can't imagine why we would allow direct reclaim or compaction to recurse into direct reclaim or compaction. > If it is safe to micro-optimise that operation then we need to make > sure that it's really really safe and that there is no risk of > accidentally breaking things later on as code evolves. > If the code evolves in that direction, it's pretty dangerous. > One way of doing that would be to add a WARN_ON(p->flags & PF_MEMALLOC) > on entry. > That would be reasonable. > Oh, and since when did we use `p' to identify task_structs? > It's pretty stupid all right but it was the name chosen for other parts of page_alloc.c. A patch that either removed all local caching or renamed p to tsk throughout page_alloc.c would be reasonable. -- 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/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: email@kvack.org