linux-kernel - Re: [PATCH v3 01/13] mm, THP: don't hold mmap

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Date:	Mon, 23 Jun 2014 13:39:29 +0800
From:	Zhang Yanfei <zhangyanfei@...fujitsu.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	Vlastimil Babka <vbabka@...e.cz>, <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan@...nel.org>, Mel Gorman <mgorman@...e.de>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 01/13] mm, THP: don't hold mmap_sem in khugepaged when
 allocating THP

Hello

On 06/21/2014 01:45 AM, Kirill A. Shutemov wrote:
> On Fri, Jun 20, 2014 at 05:49:31PM +0200, Vlastimil Babka wrote:
>> When allocating huge page for collapsing, khugepaged currently holds mmap_sem
>> for reading on the mm where collapsing occurs. Afterwards the read lock is
>> dropped before write lock is taken on the same mmap_sem.
>>
>> Holding mmap_sem during whole huge page allocation is therefore useless, the
>> vma needs to be rechecked after taking the write lock anyway. Furthemore, huge
>> page allocation might involve a rather long sync compaction, and thus block
>> any mmap_sem writers and i.e. affect workloads that perform frequent m(un)map
>> or mprotect oterations.
>>
>> This patch simply releases the read lock before allocating a huge page. It
>> also deletes an outdated comment that assumed vma must be stable, as it was
>> using alloc_hugepage_vma(). This is no longer true since commit 9f1b868a13
>> ("mm: thp: khugepaged: add policy for finding target node").
> 
> There is no point in touching ->mmap_sem in khugepaged_alloc_page() at
> all. Please, move up_read() outside khugepaged_alloc_page().
> 

I might be wrong. If we up_read in khugepaged_scan_pmd(), then if we round again
do the for loop to get the next vma and handle it. Does we do this without holding
the mmap_sem in any mode?

And if the loop end, we have another up_read in breakouterloop. What if we have
released the mmap_sem in collapse_huge_page()?

-- 
Thanks.
Zhang Yanfei
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