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

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

On 9/27/18 5:50 AM, Kirill A. Shutemov wrote:
> On Thu, Sep 27, 2018 at 02:10:34AM +0800, Yang Shi wrote:
>> brk might be used to shinrk memory mapping too other than munmap().
> s/shinrk/shrink/
>
>> 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 brk() will not manipulate vmas anymore after __do_munmap() call for
>> the mapping shrink use case. But, it may set mm->brk after
>> __do_munmap(), which needs hold write mmap_sem.
>>
>> However, a simple trick can workaround this by setting mm->brk before
>> __do_munmap(). Then restore the original value if __do_munmap() fails.
>> With this trick, 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.
>>
>> 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>
>> ---
>> v2: Rephrase the commit per Michal
>>
>>   mm/mmap.c | 40 ++++++++++++++++++++++++++++++----------
>>   1 file changed, 30 insertions(+), 10 deletions(-)
>>
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 017bcfa..0d2fae1 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -193,9 +193,11 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
>>   	unsigned long retval;
>>   	unsigned long newbrk, oldbrk;
>>   	struct mm_struct *mm = current->mm;
>> +	unsigned long origbrk = mm->brk;
> Is it safe to read mm->brk outside the lock?

Aha, thanks for catching this. It can be moved inside down_write().

Will solve in the next version.

Thanks,
Yang

>
>>   	struct vm_area_struct *next;
>>   	unsigned long min_brk;
>>   	bool populate;
>> +	bool downgrade = false;
> Again,
>
> s/downgrade/downgraded/ ?
>
>>   	LIST_HEAD(uf);
>>   
>>   	if (down_write_killable(&mm->mmap_sem))
>> @@ -229,14 +231,29 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
>>   
>>   	newbrk = PAGE_ALIGN(brk);
>>   	oldbrk = PAGE_ALIGN(mm->brk);
>> -	if (oldbrk == newbrk)
>> -		goto set_brk;
>> +	if (oldbrk == newbrk) {
>> +		mm->brk = brk;
>> +		goto success;
>> +	}
>>   
>> -	/* Always allow shrinking brk. */
>> +	/*
>> +	 * Always allow shrinking brk.
>> +	 * __do_munmap() may downgrade mmap_sem to read.
>> +	 */
>>   	if (brk <= mm->brk) {
>> -		if (!do_munmap(mm, newbrk, oldbrk-newbrk, &uf))
>> -			goto set_brk;
>> -		goto out;
>> +		/*
>> +		 * mm->brk need to be protected by write mmap_sem, update it
>> +		 * before downgrading mmap_sem.
>> +		 * When __do_munmap fail, it will be restored from origbrk.
>> +		 */
>> +		mm->brk = brk;
>> +		retval = __do_munmap(mm, newbrk, oldbrk-newbrk, &uf, true);
>> +		if (retval < 0) {
>> +			mm->brk = origbrk;
>> +			goto out;
>> +		} else if (retval == 1)
>> +			downgrade = true;
>> +		goto success;
>>   	}
>>   
>>   	/* Check against existing mmap mappings. */
>> @@ -247,18 +264,21 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
>>   	/* Ok, looks good - let it rip. */
>>   	if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0)
>>   		goto out;
>> -
>> -set_brk:
>>   	mm->brk = brk;
>> +
>> +success:
>>   	populate = newbrk > oldbrk && (mm->def_flags & VM_LOCKED) != 0;
>> -	up_write(&mm->mmap_sem);
>> +	if (downgrade)
>> +		up_read(&mm->mmap_sem);
>> +	else
>> +		up_write(&mm->mmap_sem);
>>   	userfaultfd_unmap_complete(mm, &uf);
>>   	if (populate)
>>   		mm_populate(oldbrk, newbrk - oldbrk);
>>   	return brk;
>>   
>>   out:
>> -	retval = mm->brk;
>> +	retval = origbrk;
>>   	up_write(&mm->mmap_sem);
>>   	return retval;
>>   }
>> -- 
>> 1.8.3.1
>>