linux-kernel - Re: [PATCH v3] mm: simplify find_vma

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Date:	Mon, 12 Dec 2011 14:24:43 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Shaohua Li <shaohua.li@...el.com>
Subject: Re: [PATCH v3] mm: simplify find_vma_prev

>> Why have you removed this guard? Previously we had pprev==NULL and
>> returned mm->mmap.
>> This seems like a semantic change without any explanation. Could you
>> clarify?
>
> Scratch that. I have misread the code. find_vma will return mm->mmap if
> the given address is bellow all vmas. Sorry about noise.
>
> The only concern left would be the caching. Are you sure this will not
> break some workloads which benefit from mmap_cache usage and would
> interfere with find_vma_prev callers now? Anyway this could be fixed
> trivially.

Here is callers list.

find_vma_prev     115 arch/ia64/mm/fault.c      vma =
find_vma_prev(mm, address, &prev_vma);
find_vma_prev     183 arch/parisc/mm/fault.c    vma =
find_vma_prev(mm, address, &prev_vma);
find_vma_prev     229 arch/tile/mm/hugetlbpage.c                vma =
find_vma_prev(mm, addr, &prev_vma);
find_vma_prev     336 arch/x86/mm/hugetlbpage.c                 if
(!(vma = find_vma_prev(mm, addr, &prev_vma)))
find_vma_prev     388 mm/madvise.c      vma =
find_vma_prev(current->mm, start, &prev);
find_vma_prev     642 mm/mempolicy.c    vma = find_vma_prev(mm, start, &prev);
find_vma_prev     388 mm/mlock.c        vma =
find_vma_prev(current->mm, start, &prev);
find_vma_prev     265 mm/mprotect.c     vma =
find_vma_prev(current->mm, start, &prev);

In short, find_find_prev() is only used from page fault, madvise, mbind, mlock
and mprotect. And page fault is only performance impact callsite because other
don't used frequently on regular workload.

So, I wouldn't say, this patch has zero negative impact, but I think
it is enough
small and benefit is enough much.

Thanks.
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