Re: [v2 PATCH 2/2 -mm] mm: brk: dwongrade mmap_sem to read when shrinking

[Yang Shi <yang.shi@linux.alibaba.com>]

On 9/27/18 5:14 AM, Vlastimil Babka wrote:
> On 9/26/18 8:10 PM, Yang Shi wrote:
>
> Again, "downgrade" in the subject
>
>> brk might be used to shinrk memory mapping too other than munmap().
>                         ^ shrink
>
>> So, it may hold write mmap_sem for long time when shrinking large
>> mapping, as what commit ("mm: mmap: zap pages with read mmap_sem in
>> munmap") described.
>>
>> The brk() will not manipulate vmas anymore after __do_munmap() call for
>> the mapping shrink use case. But, it may set mm->brk after
>> __do_munmap(), which needs hold write mmap_sem.
>>
>> However, a simple trick can workaround this by setting mm->brk before
>> __do_munmap(). Then restore the original value if __do_munmap() fails.
>> With this trick, it is safe to downgrade to read mmap_sem.
>>
>> So, the same optimization, which downgrades mmap_sem to read for
>> zapping pages, is also feasible and reasonable to this case.
>>
>> The period of holding exclusive mmap_sem for shrinking large mapping
>> would be reduced significantly with this optimization.
>>
>> Cc: Michal Hocko <mhocko@kernel.org>
>> Cc: Kirill A. Shutemov <kirill@shutemov.name>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>
> Same nit for the "bool downgrade" name as for patch 1/2.

Will solve in next version.

Thanks,
Yang

>
> Thanks,
> Vlastimil