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Date: Thu, 27 Sep 2018 09:04:55 -0700 From: Yang Shi <yang.shi@...ux.alibaba.com> To: Vlastimil Babka <vbabka@...e.cz>, mhocko@...nel.org, kirill@...temov.name, willy@...radead.org, ldufour@...ux.vnet.ibm.com, akpm@...ux-foundation.org Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org Subject: Re: [v2 PATCH 1/2 -mm] mm: mremap: dwongrade mmap_sem to read when shrinking On 9/27/18 4:50 AM, Vlastimil Babka wrote: > On 9/26/18 8:10 PM, Yang Shi wrote: >> Subject: [v2 PATCH 1/2 -mm] mm: mremap: dwongrade mmap_sem to read > when shrinking > > "downgrade" in the subject Will fix in the next version. Thanks, Yang > >> Other than munmap, mremap might be used to shrink memory mapping too. >> 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 mremap() will not manipulate vmas anymore after __do_munmap() call for >> the mapping shrink use case, so 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. >> >> MREMAP_FIXED and MREMAP_MAYMOVE are more complicated to adopt this >> optimization since they need manipulate vmas after do_munmap(), >> downgrading mmap_sem may create race window. >> >> Simple mapping shrink is the low hanging fruit, and it may cover the >> most cases of unmap with munmap together. >> >> Cc: Michal Hocko <mhocko@...nel.org> >> Cc: Kirill A. Shutemov <kirill@...temov.name> >> Cc: Matthew Wilcox <willy@...radead.org> >> Cc: Laurent Dufour <ldufour@...ux.vnet.ibm.com> >> Cc: Vlastimil Babka <vbabka@...e.cz> >> Cc: Andrew Morton <akpm@...ux-foundation.org> >> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com> > Looks fine, > > Acked-by: Vlastimil Babka <vbabka@...e.cz> > > Nit: > >> --- a/mm/mmap.c >> +++ b/mm/mmap.c >> @@ -2687,8 +2687,8 @@ int split_vma(struct mm_struct *mm, struct vm_area_struct *vma, >> * work. This now handles partial unmappings. >> * Jeremy Fitzhardinge <jeremy@...p.org> >> */ >> -static int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len, >> - struct list_head *uf, bool downgrade) >> +int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len, >> + struct list_head *uf, bool downgrade) >> { >> unsigned long end; >> struct vm_area_struct *vma, *prev, *last; >> diff --git a/mm/mremap.c b/mm/mremap.c >> index 5c2e185..8f1ec2b 100644 >> --- a/mm/mremap.c >> +++ b/mm/mremap.c >> @@ -525,6 +525,7 @@ static int vma_expandable(struct vm_area_struct *vma, unsigned long delta) >> unsigned long ret = -EINVAL; >> unsigned long charged = 0; >> bool locked = false; >> + bool downgrade = false; > Maybe "downgraded" is more accurate here, or even "downgraded_mmap_sem".
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