Re: [PATCH v3 01/13] mm, THP: don't hold mmap_sem in khugepaged when allocating THP

[Zhang Yanfei]

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 also deletes an outdated comment that assumed vma must be stable, as it was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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