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Date: Thu, 26 Apr 2012 14:05:11 -0700 (PDT)
From: David Rientjes <rientjes@google.com>
Subject: Re: [patch v2] thp, memcg: split hugepage for memcg oom on cow
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To: Johannes Weiner <jweiner@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Andrea Arcangeli <aarcange@redhat.com>, linux-mm@kvack.org

On Thu, 26 Apr 2012, Johannes Weiner wrote:

> > I agree it's more robust if do_huge_pmd_wp_page() were modified later and 
> > mistakenly returned VM_FAULT_OOM without the page being split, but 
> > __split_huge_page_pmd() has the drawback of also requiring to retake 
> > mm->page_table_lock to test whether orig_pmd is still legitimate so it 
> > will be slower.  Do you feel strongly about the way it's currently written 
> > which will be faster at runtime?
> 
> If you can't accomodate for a hugepage, this code runs 511 times in
> the worst case before you also can't fit a regular page anymore.  And
> compare it to the cost of the splitting itself and the subsequent 4k
> COW break faults...
> 
> I don't think it's a path worth optimizing for at all, especially if
> it includes sprinkling undocumented split_huge_pages around, and the
> fix could be as self-contained as something like this...
> 

I disagree that we should be unnecessarily taking mm->page_table_lock 
which is already strongly contended if all cpus are pagefaulting on the 
same process (and I'll be posting a patch to address specifically those 
slowdowns since thp is _much_ slower on page fault tests) when we can 
already do it in do_huge_pmd_wp_page().  If you'd like to add a comment 
for the split_huge_page() in that function if it's not clear enough from 
my VM_FAULT_OOM comment in handle_mm_fault(), then feel free to add it but 
I thought it was rather trivial to understand.

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