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

[Yang Shi <yang.shi@linux.alibaba.com>]

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@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".