linux-kernel - Re: [PATCH] mm: munlock use mapcount to avoid terrible overhead

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Date:	Tue, 18 Oct 2011 17:59:47 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: munlock use mapcount to avoid terrible overhead

On Tue, 18 Oct 2011, Andi Kleen wrote:
> Hugh Dickins <hughd@...gle.com> writes:
> 
> > A process spent 30 minutes exiting, just munlocking the pages of a large
> > anonymous area that had been alternately mprotected into page-sized vmas:
> > for every single page there's an anon_vma walk through all the other
> > little vmas to find the right one.
> 
> We had the same problem recently after a mmap+touch workload: in this
> case it was hugepaged walking all these anon_vmas and the list was over
> 100k long. 
> 
> Had some data on this at plumbers:
> http://halobates.de/plumbers-fork-locks_v2.pdf
> 
> > A general fix to that would be a lot more complicated (use prio_tree on
> > anon_vma?), but there's one very simple thing we can do to speed up the
> > common case: if a page to be munlocked is mapped only once, then it is
> > our vma that it is mapped into, and there's no need whatever to walk
> > through all the others.
> 
> I think we need a generic fix, this problem does not only happen
> in munmap. 

Thanks for the pointer, Andi, I'll have to look into it when I've a
moment; but I don't look forward to making this area more complicated.

Hugh
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