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>

1 files changed, 6 insertions, 122 deletions

diff --git a/Documentation/cgroups/memcg_test.txt b/Documentation/cgroups/memcg_test.txt
index bcf750d3cecd..8870b0212150 100644
--- a/Documentation/cgroups/memcg_test.txt
+++ b/Documentation/cgroups/memcg_test.txt
@@ -29,28 +29,13 @@ Please note that implementation details can be changed.
 2. Uncharge
   a page/swp_entry may be uncharged (usage -= PAGE_SIZE) by
 
-        mem_cgroup_uncharge_page()
-          Called when an anonymous page is fully unmapped. I.e., mapcount goes
-          to 0. If the page is SwapCache, uncharge is delayed until
-          mem_cgroup_uncharge_swapcache().
-
-        mem_cgroup_uncharge_cache_page()
-          Called when a page-cache is deleted from radix-tree. If the page is
-          SwapCache, uncharge is delayed until mem_cgroup_uncharge_swapcache().
-
-        mem_cgroup_uncharge_swapcache()
-          Called when SwapCache is removed from radix-tree. The charge itself
-          is moved to swap_cgroup. (If mem+swap controller is disabled, no
-          charge to swap occurs.)
+        mem_cgroup_uncharge()
+          Called when a page's refcount goes down to 0.
 
         mem_cgroup_uncharge_swap()
           Called when swp_entry's refcnt goes down to 0. A charge against swap
           disappears.
 
-        mem_cgroup_end_migration(old, new)
-        At success of migration old is uncharged (if necessary), a charge
-        to new page is committed. At failure, charge to old page is committed.
-
 3. charge-commit-cancel
         Memcg pages are charged in two steps:
                 mem_cgroup_try_charge()
@@ -69,18 +54,6 @@ Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
         Anonymous page is newly allocated at
                   - page fault into MAP_ANONYMOUS mapping.
                   - Copy-On-Write.
-        It is charged right after it's allocated before doing any page table
-        related operations. Of course, it's uncharged when another page is used
-        for the fault address.
-
-        At freeing anonymous page (by exit() or munmap()), zap_pte() is called
-        and pages for ptes are freed one by one.(see mm/memory.c). Uncharges
-        are done at page_remove_rmap() when page_mapcount() goes down to 0.
-
-        Another page freeing is by page-reclaim (vmscan.c) and anonymous
-        pages are swapped out. In this case, the page is marked as
-        PageSwapCache(). uncharge() routine doesn't uncharge the page marked
-        as SwapCache(). It's delayed until __delete_from_swap_cache().
 
         4.1 Swap-in.
         At swap-in, the page is taken from swap-cache. There are 2 cases.
@@ -89,41 +62,6 @@ Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
         (b) If the SwapCache has been mapped by processes, it has been
             charged already.
 
-        This swap-in is one of the most complicated work. In do_swap_page(),
-        following events occur when pte is unchanged.
-
-        (1) the page (SwapCache) is looked up.
-        (2) lock_page()
-        (3) try_charge_swapin()
-        (4) reuse_swap_page() (may call delete_swap_cache())
-        (5) commit_charge_swapin()
-        (6) swap_free().
-
-        Considering following situation for example.
-
-        (A) The page has not been charged before (2) and reuse_swap_page()
-            doesn't call delete_from_swap_cache().
-        (B) The page has not been charged before (2) and reuse_swap_page()
-            calls delete_from_swap_cache().
-        (C) The page has been charged before (2) and reuse_swap_page() doesn't
-            call delete_from_swap_cache().
-        (D) The page has been charged before (2) and reuse_swap_page() calls
-            delete_from_swap_cache().
-
-            memory.usage/memsw.usage changes to this page/swp_entry will be
-         Case          (A)      (B)       (C)     (D)
-         Event
-       Before (2)     0/ 1     0/ 1      1/ 1    1/ 1
-          ===========================================
-          (3)        +1/+1    +1/+1     +1/+1   +1/+1
-          (4)          -       0/ 0       -     -1/ 0
-          (5)         0/-1     0/ 0     -1/-1    0/ 0
-          (6)          -       0/-1       -      0/-1
-          ===========================================
-       Result         1/ 1     1/ 1      1/ 1    1/ 1
-
-       In any cases, charges to this page should be 1/ 1.
-
         4.2 Swap-out.
         At swap-out, typical state transition is below.
 
@@ -136,28 +74,20 @@ Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
             swp_entry's refcnt -= 1.
 
 
-        At (b), the page is marked as SwapCache and not uncharged.
-        At (d), the page is removed from SwapCache and a charge in page_cgroup
-        is moved to swap_cgroup.
-
         Finally, at task exit,
         (e) zap_pte() is called and swp_entry's refcnt -=1 -> 0.
-        Here, a charge in swap_cgroup disappears.
 
 5. Page Cache
         Page Cache is charged at
         - add_to_page_cache_locked().
 
-        uncharged at
-        - __remove_from_page_cache().
-
         The logic is very clear. (About migration, see below)
         Note: __remove_from_page_cache() is called by remove_from_page_cache()
         and __remove_mapping().
 
 6. Shmem(tmpfs) Page Cache
-        Memcg's charge/uncharge have special handlers of shmem. The best way
-        to understand shmem's page state transition is to read mm/shmem.c.
+        The best way to understand shmem's page state transition is to read
+        mm/shmem.c.
         But brief explanation of the behavior of memcg around shmem will be
         helpful to understand the logic.
 
@@ -170,56 +100,10 @@ Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
         It's charged when...
         - A new page is added to shmem's radix-tree.
         - A swp page is read. (move a charge from swap_cgroup to page_cgroup)
-        It's uncharged when
-        - A page is removed from radix-tree and not SwapCache.
-        - When SwapCache is removed, a charge is moved to swap_cgroup.
-        - When swp_entry's refcnt goes down to 0, a charge in swap_cgroup
-          disappears.
 
 7. Page Migration
-        One of the most complicated functions is page-migration-handler.
-        Memcg has 2 routines. Assume that we are migrating a page's contents
-        from OLDPAGE to NEWPAGE.
-
-        Usual migration logic is..
-        (a) remove the page from LRU.
-        (b) allocate NEWPAGE (migration target)
-        (c) lock by lock_page().
-        (d) unmap all mappings.
-        (e-1) If necessary, replace entry in radix-tree.
-        (e-2) move contents of a page.
-        (f) map all mappings again.
-        (g) pushback the page to LRU.
-        (-) OLDPAGE will be freed.
-
-        Before (g), memcg should complete all necessary charge/uncharge to
-        NEWPAGE/OLDPAGE.
-
-        The point is....
-        - If OLDPAGE is anonymous, all charges will be dropped at (d) because
-          try_to_unmap() drops all mapcount and the page will not be
-          SwapCache.
-
-        - If OLDPAGE is SwapCache, charges will be kept at (g) because
-          __delete_from_swap_cache() isn't called at (e-1)
-
-        - If OLDPAGE is page-cache, charges will be kept at (g) because
-          remove_from_swap_cache() isn't called at (e-1)
-
-        memcg provides following hooks.
-
-        - mem_cgroup_prepare_migration(OLDPAGE)
-          Called after (b) to account a charge (usage += PAGE_SIZE) against
-          memcg which OLDPAGE belongs to.
-
-        - mem_cgroup_end_migration(OLDPAGE, NEWPAGE)
-          Called after (f) before (g).
-          If OLDPAGE is used, commit OLDPAGE again. If OLDPAGE is already
-          charged, a charge by prepare_migration() is automatically canceled.
-          If NEWPAGE is used, commit NEWPAGE and uncharge OLDPAGE.
-
-          But zap_pte() (by exit or munmap) can be called while migration,
-          we have to check if OLDPAGE/NEWPAGE is a valid page after commit().
+
+        mem_cgroup_migrate()
 
 8. LRU
         Each memcg has its own private LRU. Now, its handling is under global