Re: [PATCH v3 6/9] mm/khugepaged: remove reuse_swap_page() usage

[Yang Shi <shy828301@gmail.com>]

On Mon, Jan 31, 2022 at 8:33 AM David Hildenbrand <david@redhat.com> wrote:
>
> reuse_swap_page() currently indicates if we can write to an anon page
> without COW. A COW is required if the page is shared by multiple
> processes (either already mapped or via swap entries) or if there is
> concurrent writeback that cannot tolerate concurrent page modifications.
>
> However, in the context of khugepaged we're not actually going to write
> to a read-only mapped page, we'll copy the page content to our newly
> allocated THP and map that THP writable. All we have to make sure
> is that the read-only mapped page we're about to copy won't get reused
> by another process sharing the page, otherwise, page content would
> get modified. But that is already guaranteed via multiple mechanisms
> (e.g., holding a reference, holding the page lock, removing the rmap after
>  copying the page).
>
> The swapcache handling was introduced in commit 10359213d05a ("mm:
> incorporate read-only pages into transparent huge pages") and it sounds
> like it merely wanted to mimic what do_swap_page() would do when trying
> to map a page obtained via the swapcache writable.
>
> As that logic is unnecessary, let's just remove it, removing the last
> user of reuse_swap_page().

Thanks for cleaning this up. I didn't spot anything wrong. You could
add Reviewed-by: Yang Shi <shy828301@gmail.com>

>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  include/trace/events/huge_memory.h |  1 -
>  mm/khugepaged.c                    | 11 -----------
>  2 files changed, 12 deletions(-)
>
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index 4fdb14a81108..d651f3437367 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -29,7 +29,6 @@
>         EM( SCAN_VMA_NULL,              "vma_null")                     \
>         EM( SCAN_VMA_CHECK,             "vma_check_failed")             \
>         EM( SCAN_ADDRESS_RANGE,         "not_suitable_address_range")   \
> -       EM( SCAN_SWAP_CACHE_PAGE,       "page_swap_cache")              \
>         EM( SCAN_DEL_PAGE_LRU,          "could_not_delete_page_from_lru")\
>         EM( SCAN_ALLOC_HUGE_PAGE_FAIL,  "alloc_huge_page_failed")       \
>         EM( SCAN_CGROUP_CHARGE_FAIL,    "ccgroup_charge_failed")        \
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 35f14d0a00a6..9da9325ab4d4 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -45,7 +45,6 @@ enum scan_result {
>         SCAN_VMA_NULL,
>         SCAN_VMA_CHECK,
>         SCAN_ADDRESS_RANGE,
> -       SCAN_SWAP_CACHE_PAGE,
>         SCAN_DEL_PAGE_LRU,
>         SCAN_ALLOC_HUGE_PAGE_FAIL,
>         SCAN_CGROUP_CHARGE_FAIL,
> @@ -682,16 +681,6 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>                         result = SCAN_PAGE_COUNT;
>                         goto out;
>                 }
> -               if (!pte_write(pteval) && PageSwapCache(page) &&
> -                               !reuse_swap_page(page)) {
> -                       /*
> -                        * Page is in the swap cache and cannot be re-used.
> -                        * It cannot be collapsed into a THP.
> -                        */
> -                       unlock_page(page);
> -                       result = SCAN_SWAP_CACHE_PAGE;
> -                       goto out;
> -               }
>
>                 /*
>                  * Isolate the page to avoid collapsing an hugepage
> --
> 2.34.1
>