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authorJohannes Weiner <hannes@cmpxchg.org>2014-08-08 14:19:22 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2014-08-08 15:57:17 -0700
commit0a31bc97c80c3fa87b32c091d9a930ac19cd0c40 (patch)
tree06dafd237309f9b8ded980eb420a5377989e2c0b /mm/migrate.c
parent00501b531c4723972aa11d6d4ebcf8d6552007c8 (diff)
downloadop-kernel-dev-0a31bc97c80c3fa87b32c091d9a930ac19cd0c40.zip
op-kernel-dev-0a31bc97c80c3fa87b32c091d9a930ac19cd0c40.tar.gz
mm: memcontrol: rewrite uncharge API
The memcg uncharging code that is involved towards the end of a page's lifetime - truncation, reclaim, swapout, migration - is impressively complicated and fragile. Because anonymous and file pages were always charged before they had their page->mapping established, uncharges had to happen when the page type could still be known from the context; as in unmap for anonymous, page cache removal for file and shmem pages, and swap cache truncation for swap pages. However, these operations happen well before the page is actually freed, and so a lot of synchronization is necessary: - Charging, uncharging, page migration, and charge migration all need to take a per-page bit spinlock as they could race with uncharging. - Swap cache truncation happens during both swap-in and swap-out, and possibly repeatedly before the page is actually freed. This means that the memcg swapout code is called from many contexts that make no sense and it has to figure out the direction from page state to make sure memory and memory+swap are always correctly charged. - On page migration, the old page might be unmapped but then reused, so memcg code has to prevent untimely uncharging in that case. Because this code - which should be a simple charge transfer - is so special-cased, it is not reusable for replace_page_cache(). But now that charged pages always have a page->mapping, introduce mem_cgroup_uncharge(), which is called after the final put_page(), when we know for sure that nobody is looking at the page anymore. For page migration, introduce mem_cgroup_migrate(), which is called after the migration is successful and the new page is fully rmapped. Because the old page is no longer uncharged after migration, prevent double charges by decoupling the page's memcg association (PCG_USED and pc->mem_cgroup) from the page holding an actual charge. The new bits PCG_MEM and PCG_MEMSW represent the respective charges and are transferred to the new page during migration. mem_cgroup_migrate() is suitable for replace_page_cache() as well, which gets rid of mem_cgroup_replace_page_cache(). However, care needs to be taken because both the source and the target page can already be charged and on the LRU when fuse is splicing: grab the page lock on the charge moving side to prevent changing pc->mem_cgroup of a page under migration. Also, the lruvecs of both pages change as we uncharge the old and charge the new during migration, and putback may race with us, so grab the lru lock and isolate the pages iff on LRU to prevent races and ensure the pages are on the right lruvec afterward. Swap accounting is massively simplified: because the page is no longer uncharged as early as swap cache deletion, a new mem_cgroup_swapout() can transfer the page's memory+swap charge (PCG_MEMSW) to the swap entry before the final put_page() in page reclaim. Finally, page_cgroup changes are now protected by whatever protection the page itself offers: anonymous pages are charged under the page table lock, whereas page cache insertions, swapin, and migration hold the page lock. Uncharging happens under full exclusion with no outstanding references. Charging and uncharging also ensure that the page is off-LRU, which serializes against charge migration. Remove the very costly page_cgroup lock and set pc->flags non-atomically. [mhocko@suse.cz: mem_cgroup_charge_statistics needs preempt_disable] [vdavydov@parallels.com: fix flags definition] Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Cc: Hugh Dickins <hughd@google.com> Cc: Tejun Heo <tj@kernel.org> Cc: Vladimir Davydov <vdavydov@parallels.com> Tested-by: Jet Chen <jet.chen@intel.com> Acked-by: Michal Hocko <mhocko@suse.cz> Tested-by: Felipe Balbi <balbi@ti.com> Signed-off-by: Vladimir Davydov <vdavydov@parallels.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/migrate.c')
-rw-r--r--mm/migrate.c38
1 files changed, 9 insertions, 29 deletions
diff --git a/mm/migrate.c b/mm/migrate.c
index be6dbf9..f78ec9b 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -780,6 +780,7 @@ static int move_to_new_page(struct page *newpage, struct page *page,
if (rc != MIGRATEPAGE_SUCCESS) {
newpage->mapping = NULL;
} else {
+ mem_cgroup_migrate(page, newpage, false);
if (remap_swapcache)
remove_migration_ptes(page, newpage);
page->mapping = NULL;
@@ -795,7 +796,6 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
{
int rc = -EAGAIN;
int remap_swapcache = 1;
- struct mem_cgroup *mem;
struct anon_vma *anon_vma = NULL;
if (!trylock_page(page)) {
@@ -821,9 +821,6 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
lock_page(page);
}
- /* charge against new page */
- mem_cgroup_prepare_migration(page, newpage, &mem);
-
if (PageWriteback(page)) {
/*
* Only in the case of a full synchronous migration is it
@@ -833,10 +830,10 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
*/
if (mode != MIGRATE_SYNC) {
rc = -EBUSY;
- goto uncharge;
+ goto out_unlock;
}
if (!force)
- goto uncharge;
+ goto out_unlock;
wait_on_page_writeback(page);
}
/*
@@ -872,7 +869,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
*/
remap_swapcache = 0;
} else {
- goto uncharge;
+ goto out_unlock;
}
}
@@ -885,7 +882,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
* the page migration right away (proteced by page lock).
*/
rc = balloon_page_migrate(newpage, page, mode);
- goto uncharge;
+ goto out_unlock;
}
/*
@@ -904,7 +901,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
VM_BUG_ON_PAGE(PageAnon(page), page);
if (page_has_private(page)) {
try_to_free_buffers(page);
- goto uncharge;
+ goto out_unlock;
}
goto skip_unmap;
}
@@ -923,10 +920,7 @@ skip_unmap:
if (anon_vma)
put_anon_vma(anon_vma);
-uncharge:
- mem_cgroup_end_migration(mem, page, newpage,
- (rc == MIGRATEPAGE_SUCCESS ||
- rc == MIGRATEPAGE_BALLOON_SUCCESS));
+out_unlock:
unlock_page(page);
out:
return rc;
@@ -1786,7 +1780,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
pg_data_t *pgdat = NODE_DATA(node);
int isolated = 0;
struct page *new_page = NULL;
- struct mem_cgroup *memcg = NULL;
int page_lru = page_is_file_cache(page);
unsigned long mmun_start = address & HPAGE_PMD_MASK;
unsigned long mmun_end = mmun_start + HPAGE_PMD_SIZE;
@@ -1852,15 +1845,6 @@ fail_putback:
goto out_unlock;
}
- /*
- * Traditional migration needs to prepare the memcg charge
- * transaction early to prevent the old page from being
- * uncharged when installing migration entries. Here we can
- * save the potential rollback and start the charge transfer
- * only when migration is already known to end successfully.
- */
- mem_cgroup_prepare_migration(page, new_page, &memcg);
-
orig_entry = *pmd;
entry = mk_pmd(new_page, vma->vm_page_prot);
entry = pmd_mkhuge(entry);
@@ -1888,14 +1872,10 @@ fail_putback:
goto fail_putback;
}
+ mem_cgroup_migrate(page, new_page, false);
+
page_remove_rmap(page);
- /*
- * Finish the charge transaction under the page table lock to
- * prevent split_huge_page() from dividing up the charge
- * before it's fully transferred to the new page.
- */
- mem_cgroup_end_migration(memcg, page, new_page, true);
spin_unlock(ptl);
mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
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