Re: [PATCH 2/7] mm/khugepaged: stop swapping in page when VM_FAULT_RETRY occurs

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/6/16 1:49, Yang Shi wrote:
> On Sat, Jun 11, 2022 at 1:47 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
>>
>> When do_swap_page returns VM_FAULT_RETRY, we do not retry here and thus
>> swap entry will remain in pagetable. This will result in later failure.
>> So stop swapping in pages in this case to save cpu cycles.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> ---
>>  mm/khugepaged.c | 19 ++++++++-----------
>>  1 file changed, 8 insertions(+), 11 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 73570dfffcec..a8adb2d1e9c6 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1003,19 +1003,16 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
>>                 swapped_in++;
>>                 ret = do_swap_page(&vmf);
>>
>> -               /* do_swap_page returns VM_FAULT_RETRY with released mmap_lock */
>> +               /*
>> +                * do_swap_page returns VM_FAULT_RETRY with released mmap_lock.
>> +                * Note we treat VM_FAULT_RETRY as VM_FAULT_ERROR here because
>> +                * we do not retry here and swap entry will remain in pagetable
>> +                * resulting in later failure.
> 
> Yeah, it makes sense.
> 
>> +                */
>>                 if (ret & VM_FAULT_RETRY) {
>>                         mmap_read_lock(mm);
> 
> A further optimization, you should not need to relock mmap_lock. You
> may consider returning a different value or passing in *locked and
> setting it to false, then check this value in the caller to skip
> unlock.

Could we just keep the mmap_sem unlocked when __collapse_huge_page_swapin() fails due to the caller
always doing mmap_read_unlock when __collapse_huge_page_swapin() returns false and add some comments
about this behavior? This looks like a simple way for me.

> 
>> -                       if (hugepage_vma_revalidate(mm, haddr, &vma)) {
>> -                               /* vma is no longer available, don't continue to swapin */
>> -                               trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
>> -                               return false;
>> -                       }
>> -                       /* check if the pmd is still valid */
>> -                       if (mm_find_pmd(mm, haddr) != pmd) {
>> -                               trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
>> -                               return false;
>> -                       }
>> +                       trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
>> +                       return false;
>>                 }
>>                 if (ret & VM_FAULT_ERROR) {
>>                         trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
> 
> And I think "swapped_in++" needs to be moved after error handling.

Do you mean do "swapped_in++" only after pages are swapped in successfully?

Thanks!

> 
>> --
>> 2.23.0
>>
>>
> .
>