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

[Vlastimil Babka <vbabka@suse.cz>]

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@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>

Looks fine,

Acked-by: Vlastimil Babka <vbabka@suse.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@goop.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".