linux-kernel - Re: [RFC][PATCH] mm: hugetlb: add stub-like do_hugetlb

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Date:	Mon, 30 Mar 2015 19:42:13 +0900
From:	Naoya Horiguchi <nao.horiguchi@...il.com>
To:	Mel Gorman <mgorman@...e.de>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] mm: hugetlb: add stub-like do_hugetlb_numa()

On 03/30/2015 07:28 PM, Mel Gorman wrote:
> On Mon, Mar 30, 2015 at 09:40:54AM +0000, Naoya Horiguchi wrote:
>> hugetlb doesn't support NUMA balancing now, but that doesn't mean that we
>> don't have to make hugetlb code prepared for PROTNONE entry properly.
>> In the current kernel, when a process accesses to hugetlb range protected
>> with PROTNONE, it causes unexpected COWs, which finally put hugetlb subsystem
>> into broken/uncontrollable state, where for example h->resv_huge_pages is
>> subtracted too much and wrapped around to a very large number, and free
>> hugepage pool is no longer maintainable.
>>
>
> Ouch!
>
>> This patch simply clears PROTNONE when it's caught out. Real NUMA balancing
>> code for hugetlb is not implemented yet (not sure how much it's worth doing.)
>>
>
> It's not worth doing at all. Furthermore, an application that took the
> effort to allocate and use hugetlb pages is not going to appreciate the
> minor faults incurred by automatic balancing for no gain.

OK,

> Why not something
> like the following untested patch?

I'll test this tomorrow.
Thank you very much for the comment.

Naoya Horiguchi

> It simply avoids doing protection updates
> on hugetlb VMAs. If it works for you, feel free to take it and reuse most
> of the same changelog for it. I'll only be intermittently online for the
> next few days and would rather not unnecessarily delay a fix
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7ce18f3c097a..74bfde50fd4e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2161,8 +2161,10 @@ void task_numa_work(struct callback_head *work)
>   		vma = mm->mmap;
>   	}
>   	for (; vma; vma = vma->vm_next) {
> -		if (!vma_migratable(vma) || !vma_policy_mof(vma))
> +		if (!vma_migratable(vma) || !vma_policy_mof(vma) ||
> +						is_vm_hugetlb_page(vma)) {
>   			continue;
> +		}
>
>   		/*
>   		 * Shared library pages mapped by multiple processes are not
>
> --
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