Re: [PATCH] mm: unmapped page migration avoid unmap+remap overhead

[Yasuaki Ishimatsu]

(2014/12/01 13:52), Hugh Dickins wrote:
> Page migration's __unmap_and_move(), and rmap's try_to_unmap(),
> were created for use on pages almost certainly mapped into userspace.
> But nowadays compaction often applies them to unmapped page cache pages:
> which may exacerbate contention on i_mmap_rwsem quite unnecessarily,
> since try_to_unmap_file() makes no preliminary page_mapped() check.
>
> Now check page_mapped() in __unmap_and_move(); and avoid repeating the
> same overhead in rmap_walk_file() - don't remove_migration_ptes() when
> we never inserted any.
>
> (The PageAnon(page) comment blocks now look even sillier than before,
> but clean that up on some other occasion.  And note in passing that
> try_to_unmap_one() does not use a migration entry when PageSwapCache,
> so remove_migration_ptes() will then not update that swap entry to
> newpage pte: not a big deal, but something else to clean up later.)
>
> Davidlohr remarked in "mm,fs: introduce helpers around the i_mmap_mutex"
> conversion to i_mmap_rwsem, that "The biggest winner of these changes
> is migration": a part of the reason might be all of that unnecessary
> taking of i_mmap_mutex in page migration; and it's rather a shame that
> I didn't get around to sending this patch in before his - this one is
> much less useful after Davidlohr's conversion to rwsem, but still good.
>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
>
>   mm/migrate.c |   28 ++++++++++++++++++----------
>   1 file changed, 18 insertions(+), 10 deletions(-)
>
> --- 3.18-rc7/mm/migrate.c	2014-10-19 22:12:56.809625067 -0700
> +++ linux/mm/migrate.c	2014-11-30 20:17:51.205187663 -0800
> @@ -746,7 +746,7 @@ static int fallback_migrate_page(struct
>    *  MIGRATEPAGE_SUCCESS - success
>    */
>   static int move_to_new_page(struct page *newpage, struct page *page,
> -				int remap_swapcache, enum migrate_mode mode)
> +				int page_was_mapped, enum migrate_mode mode)
>   {
>   	struct address_space *mapping;
>   	int rc;
> @@ -784,7 +784,7 @@ static int move_to_new_page(struct page
>   		newpage->mapping = NULL;
>   	} else {
>   		mem_cgroup_migrate(page, newpage, false);
> -		if (remap_swapcache)
> +		if (page_was_mapped)
>   			remove_migration_ptes(page, newpage);
>   		page->mapping = NULL;
>   	}
> @@ -798,7 +798,7 @@ static int __unmap_and_move(struct page
>   				int force, enum migrate_mode mode)
>   {
>   	int rc = -EAGAIN;
> -	int remap_swapcache = 1;
> +	int page_was_mapped = 0;
>   	struct anon_vma *anon_vma = NULL;
>
>   	if (!trylock_page(page)) {
> @@ -870,7 +870,6 @@ static int __unmap_and_move(struct page
>   			 * migrated but are not remapped when migration
>   			 * completes
>   			 */
> -			remap_swapcache = 0;
>   		} else {
>   			goto out_unlock;
>   		}
> @@ -910,13 +909,17 @@ static int __unmap_and_move(struct page
>   	}
>
>   	/* Establish migration ptes or remove ptes */

> -	try_to_unmap(page, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
> +	if (page_mapped(page)) {
> +		try_to_unmap(page,
> +			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
> +		page_was_mapped = 1;
> +	}

Is there no possibility that page is swap cache? If page is swap cache,
this code changes behavior of move_to_new_page(). Is it O.K.?

Thanks,
Yasuaki Ishimatsu

>   skip_unmap:
>   	if (!page_mapped(page))
> -		rc = move_to_new_page(newpage, page, remap_swapcache, mode);
> +		rc = move_to_new_page(newpage, page, page_was_mapped, mode);
>
> -	if (rc && remap_swapcache)
> +	if (rc && page_was_mapped)
>   		remove_migration_ptes(page, page);
>
>   	/* Drop an anon_vma reference if we took one */
> @@ -1017,6 +1020,7 @@ static int unmap_and_move_huge_page(new_
>   {
>   	int rc = 0;
>   	int *result = NULL;
> +	int page_was_mapped = 0;
>   	struct page *new_hpage;
>   	struct anon_vma *anon_vma = NULL;
>
> @@ -1047,12 +1051,16 @@ static int unmap_and_move_huge_page(new_
>   	if (PageAnon(hpage))
>   		anon_vma = page_get_anon_vma(hpage);
>
> -	try_to_unmap(hpage, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
> +	if (page_mapped(hpage)) {
> +		try_to_unmap(hpage,
> +			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
> +		page_was_mapped = 1;
> +	}
>
>   	if (!page_mapped(hpage))
> -		rc = move_to_new_page(new_hpage, hpage, 1, mode);
> +		rc = move_to_new_page(new_hpage, hpage, page_was_mapped, mode);
>
> -	if (rc != MIGRATEPAGE_SUCCESS)
> +	if (rc != MIGRATEPAGE_SUCCESS && page_was_mapped)
>   		remove_migration_ptes(hpage, hpage);
>
>   	if (anon_vma)
>
> --
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