linux-kernel - Re: [PATCH v2] mm/THP: use hugepage_vma_check() in khugepaged_enter_vma

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Message-ID: <8F62086E-03E5-4A65-99F3-E4ABB4FFFC70@fb.com>
Date:   Mon, 28 May 2018 18:04:00 +0000
From:   Song Liu <songliubraving@...com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Kernel Team <Kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "rientjes@...gle.com" <rientjes@...gle.com>,
        "aarcange@...hat.com" <aarcange@...hat.com>
Subject: Re: [PATCH v2] mm/THP: use hugepage_vma_check() in
 khugepaged_enter_vma_merge()



> On May 28, 2018, at 3:57 AM, Kirill A. Shutemov <kirill@...temov.name> wrote:
> 
> On Tue, May 22, 2018 at 12:44:30PM -0700, 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(). Here is an example of failed madvise():
>> 
>>   /* mount /dev/shm with huge=advise:
>>    *     mount -o remount,huge=advise /dev/shm */
>>   /* create file /dev/shm/huge */
>>   #define HUGE_FILE "/dev/shm/huge"
>> 
>>   fd = open(HUGE_FILE, O_RDONLY);
>>   ptr = mmap(NULL, FILE_SIZE, PROT_READ, MAP_PRIVATE, fd, 0);
>>   ret = madvise(ptr, FILE_SIZE, MADV_HUGEPAGE);
>> 
>> madvise() will return 0, but this memory region is never put in huge
>> page (check from /proc/meminfo: ShmemHugePages).
>> 
>> This patch fixes these problems by reusing hugepage_vma_check() in
>> khugepaged_enter_vma_merge().
>> 
>> vma->vm_flags is not yet updated in khugepaged_enter_vma_merge(),
>> so we need to pass the new vm_flags to hugepage_vma_check() through
>> a separate argument.
>> 
>> Signed-off-by: Song Liu <songliubraving@...com>
>> ---
>> mm/khugepaged.c | 26 ++++++++++++--------------
>> 1 file changed, 12 insertions(+), 14 deletions(-)
>> 
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index d7b2a4b..9f74e51 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -430,18 +430,15 @@ int __khugepaged_enter(struct mm_struct *mm)
>> 	return 0;
>> }
>> 
>> +static bool hugepage_vma_check(struct vm_area_struct *vma,
>> +			       unsigned long vm_flags);
>> +
> 
> The patch looks good to me.
> 
> But can we move hugepage_vma_check() here to avoid forward declaration of
> the function?

Thanks for the feedback! I will send v3 with this change. 

Song