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Date: Thu, 18 Mar 2010 09:48:08 +0900 (JST) From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> To: Mel Gorman <mel@....ul.ie> Cc: kosaki.motohiro@...fujitsu.com, Minchan Kim <minchan.kim@...il.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>, Andrew Morton <akpm@...ux-foundation.org>, Andrea Arcangeli <aarcange@...hat.com>, Christoph Lameter <cl@...ux-foundation.org>, Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>, David Rientjes <rientjes@...gle.com>, Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages > > > + /* > > > + * If the page has no mappings any more, just bail. An > > > + * unmapped anon page is likely to be freed soon but worse, > > > + * it's possible its anon_vma disappeared between when > > > + * the page was isolated and when we reached here while > > > + * the RCU lock was not held > > > + */ > > > + if (!page_mapcount(page)) { > > > + rcu_read_unlock(); > > > + goto uncharge; > > > + } > > > > I haven't understand what prevent this check. Why don't we need following scenario? > > > > 1. Page isolated for migration > > 2. Passed this if (!page_mapcount(page)) check > > 3. Process exits > > 4. page_mapcount(page) drops to zero so anon_vma was no longer reliable > > > > Traditionally, page migration logic is, it can touch garbarge of anon_vma, but > > SLAB_DESTROY_BY_RCU prevent any disaster. Is this broken concept? > > The check is made within the RCU read lock. If the count is positive at > that point but goes to zero due to a process exiting, the anon_vma will > still be valid until rcu_read_unlock() is called. Thank you! then, this logic depend on SLAB_DESTROY_BY_RCU, not refcount. So, I think we don't need your [1/11] patch. Am I missing something? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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