linux-kernel - Re: [patch] mm: reduce pagetable-freeing latencies

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Message-ID: <Pine.LNX.4.64.0707261758280.370@blonde.wat.veritas.com>
Date:	Thu, 26 Jul 2007 18:11:32 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [patch] mm: reduce pagetable-freeing latencies

On Wed, 25 Jul 2007, Benjamin Herrenschmidt wrote:
> 
> In fact, i discussed with peter and I think the best is to only have one
> entry in the stack mmu_gather structure.

Four or eight would be much nicer than one, but you're right there's
a stackdepth worry: good spotting of those unmap_mapping_range cases,
I'd certainly been thinking we were fairly near the top of the stack
in unmap_vmas, which is not so (or not obviously so) in those cases.

> If we fail to allocate a page
> to batch entries, we flush that one and steal it :-) Then we have a page
> to gather more.

That will often be the case for anonymous pages, but not for pagecache
pages.  I had wondered whether to make exit_mmap unmap the stack first
(rather than text first as usually happens), but seems rather a hack.

Hugh
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