linux-kernel - Re: [PATCH] mm: thp: fix false negative of shmem vma's THP eligibility

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Date:   Tue, 23 Apr 2019 09:30:44 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     vbabka@...e.cz, rientjes@...gle.com, kirill@...temov.name,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: thp: fix false negative of shmem vma's THP
 eligibility



On 4/22/19 11:50 PM, Michal Hocko wrote:
> On Tue 23-04-19 06:24:53, Yang Shi wrote:
>> The commit 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each
>> vma") introduced THPeligible bit for processes' smaps. But, when checking
>> the eligibility for shmem vma, __transparent_hugepage_enabled() is
>> called to override the result from shmem_huge_enabled().  It may result
>> in the anonymous vma's THP flag override shmem's.
> Hmm, I was under impression that thw global sysfs is not anonymous
> memory specific and it overrides whatever sysfs comes with. Isn't
> ignoring the global setting a bug in the shmemfs allocation paths?
> Kirill what is the actual semantic here?

I tried 4.9, 4.14, 4.20 and 5.1-rc5, all behaves in the same way. So, 
I'm supposed "enabled" is for anonymous THP only.

>
>> For example, running a
>> simple test which create THP for shmem, but with anonymous THP disabled,
>> when reading the process's smaps, it may show:
>>
>> 7fc92ec00000-7fc92f000000 rw-s 00000000 00:14 27764 /dev/shm/test
>> Size:               4096 kB
>> ...
>> [snip]
>> ...
>> ShmemPmdMapped:     4096 kB
>> ...
>> [snip]
>> ...
>> THPeligible:    0
>>
>> And, /proc/meminfo does show THP allocated and PMD mapped too:
>>
>> ShmemHugePages:     4096 kB
>> ShmemPmdMapped:     4096 kB
>>
>> This doesn't make too much sense.  The anonymous THP flag should not
>> intervene shmem THP.  Calling shmem_huge_enabled() with checking
>> MMF_DISABLE_THP sounds good enough.  And, we could skip stack and
>> dax vma check since we already checked if the vma is shmem already.
> Even if I am wrong about the /sys/kernel/mm/transparent_hugepage/enabled
> being the global setting for _all_ THP then this patch is not sufficient
> because it doesn't reflect VM_NOHUGEPAGE.

Aha, yes, thanks for catching this. Will fix in v2.

>> Fixes: 7635d9cbe832 ("mm, thp, proc: report THP eligibility for each vma")
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Vlastimil Babka <vbabka@...e.cz>
>> Cc: David Rientjes <rientjes@...gle.com>
>> Cc: Kirill A. Shutemov <kirill@...temov.name>
>> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
>> ---
>>   mm/huge_memory.c | 4 ++--
>>   mm/shmem.c       | 2 ++
>>   2 files changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 165ea46..5881e82 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -67,8 +67,8 @@ bool transparent_hugepage_enabled(struct vm_area_struct *vma)
>>   {
>>   	if (vma_is_anonymous(vma))
>>   		return __transparent_hugepage_enabled(vma);
>> -	if (vma_is_shmem(vma) && shmem_huge_enabled(vma))
>> -		return __transparent_hugepage_enabled(vma);
>> +	if (vma_is_shmem(vma))
>> +		return shmem_huge_enabled(vma);
>>   
>>   	return false;
>>   }
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 2275a0f..be15e9b 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -3873,6 +3873,8 @@ bool shmem_huge_enabled(struct vm_area_struct *vma)
>>   	loff_t i_size;
>>   	pgoff_t off;
>>   
>> +	if (test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
>> +		return false;
>>   	if (shmem_huge == SHMEM_HUGE_FORCE)
>>   		return true;
>>   	if (shmem_huge == SHMEM_HUGE_DENY)
>> -- 
>> 1.8.3.1
>>