[PATCH 12/12] mm: migrate dirty page without clear_page_dirty_for_io etc

Hugh Dickins
2015-10-19T05:11:21Z

clear_page_dirty_for_io() has accumulated writeback and memcg subtleties
since v2.6.16 first introduced page migration; and the set_page_dirty()
which completed its migration of PageDirty, later had to be moderated to
__set_page_dirty_nobuffers(); then PageSwapBacked had to skip that too.

No actual problems seen with this procedure recently, but if you look
into what the clear_page_dirty_for_io(page)+set_page_dirty(newpage) is
actually achieving, it turns out to be nothing more than moving the
PageDirty flag, and its NR_FILE_DIRTY stat from one zone to another.

It would be good to avoid a pile of irrelevant decrementations and
incrementations, and improper event counting, and unnecessary descent
of the radix_tree under tree_lock (to set the PAGECACHE_TAG_DIRTY which
radix_tree_replace_slot() left in place anyway).

Do the NR_FILE_DIRTY movement, like the other stats movements, while
interrupts still disabled in migrate_page_move_mapping(); and don't even
bother if the zone is the same. Do the PageDirty movement there under
tree_lock too, where old page is frozen and newpage not yet visible:
bearing in mind that as soon as newpage becomes visible in radix_tree,
an un-page-locked set_page_dirty() might interfere (or perhaps that's
just not possible: anything doing so should already hold an additional
reference to the old page, preventing its migration; but play safe).

But we do still need to transfer PageDirty in migrate_page_copy(), for
those who don't go the mapping route through migrate_page_move_mapping().

Signed-off-by: Hugh Dickins <hughd@google.com>
---
mm/migrate.c | 51 +++++++++++++++++++++++++++++--------------------
1 file changed, 31 insertions(+), 20 deletions(-)

--- migrat.orig/mm/migrate.c 2015-10-18 17:53:31.871345704 -0700
+++ migrat/mm/migrate.c 2015-10-18 17:53:33.899348014 -0700
@@ -30,6 +30,7 @@
#include <linux/mempolicy.h>
#include <linux/vmalloc.h>
#include <linux/security.h>
+#include <linux/backing-dev.h>
#include <linux/syscalls.h>
#include <linux/hugetlb.h>
#include <linux/hugetlb_cgroup.h>
@@ -313,6 +314,8 @@ int migrate_page_move_mapping(struct add
struct buffer_head *head, enum migrate_mode mode,
int extra_count)
{
+ struct zone *oldzone, *newzone;
+ int dirty;
int expected_count = 1 + extra_count;
void **pslot;

@@ -331,6 +334,9 @@ int migrate_page_move_mapping(struct add
return MIGRATEPAGE_SUCCESS;
}

+ oldzone = page_zone(page);
+ newzone = page_zone(newpage);
+
spin_lock_irq(&mapping->tree_lock);

pslot = radix_tree_lookup_slot(&mapping->page_tree,
@@ -378,6 +384,13 @@ int migrate_page_move_mapping(struct add
set_page_private(newpage, page_private(page));
}

+ /* Move dirty while page refs frozen and newpage not yet exposed */
+ dirty = PageDirty(page);
+ if (dirty) {
+ ClearPageDirty(page);
+ SetPageDirty(newpage);
+ }
+
radix_tree_replace_slot(pslot, newpage);

/*
@@ -387,6 +400,9 @@ int migrate_page_move_mapping(struct add
*/
page_unfreeze_refs(page, expected_count - 1);

+ spin_unlock(&mapping->tree_lock);
+ /* Leave irq disabled to prevent preemption while updating stats */
+
/*
* If moved to a different zone then also account
* the page for that zone. Other VM counters will be
@@ -397,13 +413,19 @@ int migrate_page_move_mapping(struct add
* via NR_FILE_PAGES and NR_ANON_PAGES if they
* are mapped to swap space.
*/
- __dec_zone_page_state(page, NR_FILE_PAGES);
- __inc_zone_page_state(newpage, NR_FILE_PAGES);
- if (!PageSwapCache(page) && PageSwapBacked(page)) {
- __dec_zone_page_state(page, NR_SHMEM);
- __inc_zone_page_state(newpage, NR_SHMEM);
+ if (newzone != oldzone) {
+ __dec_zone_state(oldzone, NR_FILE_PAGES);
+ __inc_zone_state(newzone, NR_FILE_PAGES);
+ if (PageSwapBacked(page) && !PageSwapCache(page)) {
+ __dec_zone_state(oldzone, NR_SHMEM);
+ __inc_zone_state(newzone, NR_SHMEM);
+ }
+ if (dirty && mapping_cap_account_dirty(mapping)) {
+ __dec_zone_state(oldzone, NR_FILE_DIRTY);
+ __inc_zone_state(newzone, NR_FILE_DIRTY);
+ }
}
- spin_unlock_irq(&mapping->tree_lock);
+ local_irq_enable();

return MIGRATEPAGE_SUCCESS;
}
@@ -524,20 +546,9 @@ void migrate_page_copy(struct page *newp
if (PageMappedToDisk(page))
SetPageMappedToDisk(newpage);

- if (PageDirty(page)) {
- clear_page_dirty_for_io(page);
- /*
- * Want to mark the page and the radix tree as dirty, and
- * redo the accounting that clear_page_dirty_for_io undid,
- * but we can't use set_page_dirty because that function
- * is actually a signal that all of the page has become dirty.
- * Whereas only part of our page may be dirty.
- */
- if (PageSwapBacked(page))
- SetPageDirty(newpage);
- else
- __set_page_dirty_nobuffers(newpage);
- }
+ /* Move dirty on pages not done by migrate_page_move_mapping() */
+ if (PageDirty(page))
+ SetPageDirty(newpage);

if (page_is_young(page))
set_page_young(newpage);