Messages in this thread | | | Subject | Re: [v2 PATCH 2/2 -mm] mm: brk: dwongrade mmap_sem to read when shrinking | From | Vlastimil Babka <> | Date | Thu, 27 Sep 2018 14:14:00 +0200 |
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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.
Thanks, Vlastimil
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