Re: [v3 PATCH 2/2] mm: thp: fix false negative of shmem vma's THP eligibility

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

On 6/19/19 5:12 AM, Vlastimil Babka wrote:
> On 6/13/19 6:44 AM, 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.  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:
> ...
>
>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index 01d4eb0..6a13882 100644
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -796,7 +796,8 @@ static int show_smap(struct seq_file *m, void *v)
>>   
>>   	__show_smap(m, &mss);
>>   
>> -	seq_printf(m, "THPeligible:    %d\n", transparent_hugepage_enabled(vma));
>> +	seq_printf(m, "THPeligible:		%d\n",
>> +		   transparent_hugepage_enabled(vma));
>>   
>>   	if (arch_pkeys_enabled())
>>   		seq_printf(m, "ProtectionKey:  %8u\n", vma_pkey(vma));
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 4bc2552..36f0225 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -65,10 +65,15 @@
>>   
>>   bool transparent_hugepage_enabled(struct vm_area_struct *vma)
>>   {
>> +	/* The addr is used to check if the vma size fits */
>> +	unsigned long addr = (vma->vm_end & HPAGE_PMD_MASK) - HPAGE_PMD_SIZE;
>> +
>> +	if (!transhuge_vma_suitable(vma, addr))
>> +		return false;
> Sorry for replying rather late, and not in the v2 thread, but unlike
> Hugh I'm not convinced that we should include vma size/alignment in the
> test for reporting THPeligible, which was supposed to reflect
> administrative settings and madvise hints. I guess it's mostly a matter
> of personal feeling. But one objective distinction is that the admin
> settings and madvise do have an exact binary result for the whole VMA,
> while this check is more fuzzy - only part of the VMA's span might be
> properly sized+aligned, and THPeligible will be 1 for the whole VMA.

I think THPeligible is used to tell us if the vma is suitable for 
allocating THP. Both anonymous and shmem THP checks vma size/alignment 
to decide to or not to allocate THP.

And, if vma size/alignment is not checked, THPeligible may show "true" 
for even 4K mapping. This doesn't make too much sense either.

>
>>   	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 1bb3b8d..a807712 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -3872,6 +3872,9 @@ bool shmem_huge_enabled(struct vm_area_struct *vma)
>>   	loff_t i_size;
>>   	pgoff_t off;
>>   
>> +	if ((vma->vm_flags & VM_NOHUGEPAGE) ||
>> +	    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)
>>