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

[Kirill A. Shutemov]

On Thu, Sep 27, 2018 at 02:10:33AM +0800, Yang Shi wrote:
> 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>
> ---
> v2: Rephrase the commit log per Michal
> 
>  include/linux/mm.h |  2 ++
>  mm/mmap.c          |  4 ++--
>  mm/mremap.c        | 17 +++++++++++++----
>  3 files changed, 17 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a61ebe8..3028028 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2286,6 +2286,8 @@ extern unsigned long do_mmap(struct file *file, unsigned long addr,
>  	unsigned long len, unsigned long prot, unsigned long flags,
>  	vm_flags_t vm_flags, unsigned long pgoff, unsigned long *populate,
>  	struct list_head *uf);
> +extern int __do_munmap(struct mm_struct *, unsigned long, size_t,
> +		       struct list_head *uf, bool downgrade);
>  extern int do_munmap(struct mm_struct *, unsigned long, size_t,
>  		     struct list_head *uf);
>  
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 847a17d..017bcfa 100644
> --- 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;

s/downgrade/downgraded/ ?

Otherwise looks good to me:

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov