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Date: Mon, 1 Dec 2014 15:44:30 +0900
From: Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
To: Hugh Dickins <hughd@...gle.com>
CC: Andrew Morton <akpm@...ux-foundation.org>,
Davidlohr Bueso <dave@...olabs.net>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: unmapped page migration avoid unmap+remap overhead
(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@...gle.com>
> ---
>
> 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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