On 2022/6/16 1:49, Yang Shi wrote:
> On Sat, Jun 11, 2022 at 1:47 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> 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@xxxxxxxxxx>
>> ---
>> 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
>>
>>
> .
>
