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

[Vlastimil Babka]

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@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Same nit for the "bool downgrade" name as for patch 1/2.

Thanks,
Vlastimil