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
> 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".
