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Date: Thu, 6 May 2010 16:51:43 +0100 From: Mel Gorman <mel@....ul.ie> To: Rik van Riel <riel@...hat.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>, Minchan Kim <minchan.kim@...il.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>, Christoph Lameter <cl@...ux.com>, Andrea Arcangeli <aarcange@...hat.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org> Subject: Re: [PATCH 1/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information On Thu, May 06, 2010 at 11:44:57AM -0400, Rik van Riel wrote: > On 05/06/2010 11:33 AM, Mel Gorman wrote: > >> @@ -1368,16 +1424,25 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *, >> * are holding mmap_sem. Users without mmap_sem are required to >> * take a reference count to prevent the anon_vma disappearing >> */ >> - anon_vma = page_anon_vma(page); >> + anon_vma = page_anon_vma_lock_root(page); >> if (!anon_vma) >> return ret; >> - spin_lock(&anon_vma->lock); >> list_for_each_entry(avc,&anon_vma->head, same_anon_vma) { > > One conceptual problem here. By taking the oldest anon_vma, > instead of the anon_vma of the page, we may end up searching > way too many processes. > > Eg. if the page is the page of a child process in a forking > server workload, the above code will end up searching the > parent and all of the siblings - even for a private page, in > the child process's private anon_vma. > > For an Apache or Oracle system with 1000 clients (and child > processes), that could be quite a drag - searching 1000 times > as many processes as we should. > That does indeed suck. If we were always locking the root anon_vma, we'd get away with it but that would involve introducing RCU into the munmap/mmap/etc path. Is there any way around this problem or will migration just have to take it on the chin until anon_vma is reference counted and we can cheaply lock the root anon_vma? -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- 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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