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
>
