On Mon, Jan 31, 2022 at 8:33 AM David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxxx>
>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
> 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
>
