On May 22, 2018, at 5:13 AM, Michal Hocko <mhocko@xxxxxxxxxx> 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@xxxxxx>
>> ---
>> 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
