Re: [v2 PATCH 1/2 -mm] mm: mremap: dwongrade mmap_sem to read when shrinking

[Yang Shi]

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@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>
> Looks fine,
>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
>
> 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@xxxxxxxx>
> >    */
> > -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".