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

[Yang Shi]

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@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.

Will solve in next version.

Thanks,
Yang

> Thanks,
> Vlastimil