Re: [PATCH] mm/THP: use hugepage_vma_check() in khugepaged_enter_vma_merge()

[Song Liu <songliubraving@fb.com>]

On May 22, 2018, at 5:13 AM, Michal Hocko <mhocko@kernel.org> wrote:
> 
> [CC Kirill]
> 
> On Mon 21-05-18 12:38:53, Song Liu wrote:
>> khugepaged_enter_vma_merge() is using a different approach to check
>> whether a vma is valid for khugepaged_enter():
>> 
>>    if (!vma->anon_vma)
>>            /*
>>             * Not yet faulted in so we will register later in the
>>             * page fault if needed.
>>             */
>>            return 0;
>>    if (vma->vm_ops || (vm_flags & VM_NO_KHUGEPAGED))
>>            /* khugepaged not yet working on file or special mappings */
>>            return 0;
>> 
>> This check has some problems. One of the obvious problems is that
>> it doesn't check shmem_file(), so that vma backed with shmem files
>> will not call khugepaged_enter().
>> 
>> This patch fixes these problems by reusing hugepage_vma_check() in
>> khugepaged_enter_vma_merge().
> 
> It would be great to be more explicit about what are the actual
> consequences. khugepaged_enter_vma_merge is called from multiple
> context. Some of then do not really care about !anon case (e.g. stack
> expansion). hugepage_madvise is quite convoluted so I am not really sure
> from a quick look (are we simply not going to merge vmas even if we
> could?).

Yes, it does fix madvise for shmem with huge=advise option. I had made
a mistake in this version. I will send v2 with the more details on what
is fixed. 

> Have you noticed this by a code inspection or you have seen this
> happening in real workloads (aka, is this worth backporting to stable
> trees)?

I noticed this when reading the code. I think this might worth back 
porting. However, I don't know whether it fixes anything else other
than shmem, so I am not sure which versions need this fix. 

Thanks,
Song


>> Signed-off-by: Song Liu <songliubraving@fb.com>
>> ---
>> mm/khugepaged.c | 12 ++++--------
>> 1 file changed, 4 insertions(+), 8 deletions(-)
>> 
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index d7b2a4b..e50c2bd 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -430,18 +430,14 @@ int __khugepaged_enter(struct mm_struct *mm)
>> 	return 0;
>> }
>> 
>> +static bool hugepage_vma_check(struct vm_area_struct *vma);
>> +
>> int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
>> 			       unsigned long vm_flags)
>> {
>> 	unsigned long hstart, hend;
>> -	if (!vma->anon_vma)
>> -		/*
>> -		 * Not yet faulted in so we will register later in the
>> -		 * page fault if needed.
>> -		 */
>> -		return 0;
>> -	if (vma->vm_ops || (vm_flags & VM_NO_KHUGEPAGED))
>> -		/* khugepaged not yet working on file or special mappings */
>> +
>> +	if (!hugepage_vma_check(vma))
>> 		return 0;
>> 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
>> 	hend = vma->vm_end & HPAGE_PMD_MASK;
>> -- 
>> 2.9.5
> 
> -- 
> Michal Hocko
> SUSE Labs