Re: [PATCH v3 3/9] mm: slightly clarify KSM logic in do_swap_page()

[Yang Shi]

On Mon, Jan 31, 2022 at 8:33 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> Let's make it clearer that KSM might only have to copy a page
> in case we have a page in the swapcache, not if we allocated a fresh
> page and bypassed the swapcache. While at it, add a comment why this is
> usually necessary and merge the two swapcache conditions.
>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  mm/memory.c | 38 +++++++++++++++++++++++---------------
>  1 file changed, 23 insertions(+), 15 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 923165b4c27e..3c91294cca98 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3615,21 +3615,29 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>                 goto out_release;
>         }
>
> -       /*
> -        * Make sure try_to_free_swap or reuse_swap_page or swapoff did not

We could remove the reference to "reuse_swap_page", right?

> -        * release the swapcache from under us.  The page pin, and pte_same
> -        * test below, are not enough to exclude that.  Even if it is still
> -        * swapcache, we need to check that the page's swap has not changed.
> -        */
> -       if (unlikely((!PageSwapCache(page) ||
> -                       page_private(page) != entry.val)) && swapcache)
> -               goto out_page;
> -
> -       page = ksm_might_need_to_copy(page, vma, vmf->address);
> -       if (unlikely(!page)) {
> -               ret = VM_FAULT_OOM;
> -               page = swapcache;
> -               goto out_page;
> +       if (swapcache) {
> +               /*
> +                * Make sure try_to_free_swap or reuse_swap_page or swapoff did
> +                * not release the swapcache from under us.  The page pin, and
> +                * pte_same test below, are not enough to exclude that.  Even if
> +                * it is still swapcache, we need to check that the page's swap
> +                * has not changed.
> +                */
> +               if (unlikely(!PageSwapCache(page) ||
> +                            page_private(page) != entry.val))
> +                       goto out_page;
> +
> +               /*
> +                * KSM sometimes has to copy on read faults, for example, if
> +                * page->index of !PageKSM() pages would be nonlinear inside the
> +                * anon VMA -- PageKSM() is lost on actual swapout.
> +                */
> +               page = ksm_might_need_to_copy(page, vma, vmf->address);
> +               if (unlikely(!page)) {
> +                       ret = VM_FAULT_OOM;
> +                       page = swapcache;
> +                       goto out_page;
> +               }
>         }
>
>         cgroup_throttle_swaprate(page, GFP_KERNEL);
> --
> 2.34.1
>