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.
> - 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.
> --
> 2.23.0
>
>
