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+Memory Resource Controller(Memcg)  Implementation Memo.
+Last Updated: 2008/12/15
+Base Kernel Version: based on 2.6.28-rc8-mm.
+
+Because VM is getting complex (one of reasons is memcg...), memcg's behavior
+is complex. This is a document for memcg's internal behavior.
+Please note that implementation details can be changed.
+
+(*) Topics on API should be in Documentation/cgroups/memory.txt)
+
+0. How to record usage ?
+   2 objects are used.
+
+   page_cgroup ....an object per page.
+        Allocated at boot or memory hotplug. Freed at memory hot removal.
+
+   swap_cgroup ... an entry per swp_entry.
+        Allocated at swapon(). Freed at swapoff().
+
+   The page_cgroup has USED bit and double count against a page_cgroup never
+   occurs. swap_cgroup is used only when a charged page is swapped-out.
+
+1. Charge
+
+   a page/swp_entry may be charged (usage += PAGE_SIZE) at
+
+        mem_cgroup_newpage_charge()
+          Called at new page fault and Copy-On-Write.
+
+        mem_cgroup_try_charge_swapin()
+          Called at do_swap_page() (page fault on swap entry) and swapoff.
+          Followed by charge-commit-cancel protocol. (With swap accounting)
+          At commit, a charge recorded in swap_cgroup is removed.
+
+        mem_cgroup_cache_charge()
+          Called at add_to_page_cache()
+
+        mem_cgroup_cache_charge_swapin()
+          Called at shmem's swapin.
+
+        mem_cgroup_prepare_migration()
+          Called before migration. "extra" charge is done and followed by
+          charge-commit-cancel protocol.
+          At commit, charge against oldpage or newpage will be committed.
+
+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_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
+        In some case, we can't know this "charge" is valid or not at charging
+        (because of races).
+        To handle such case, there are charge-commit-cancel functions.
+                mem_cgroup_try_charge_XXX
+                mem_cgroup_commit_charge_XXX
+                mem_cgroup_cancel_charge_XXX
+        these are used in swap-in and migration.
+
+        At try_charge(), there are no flags to say "this page is charged".
+        at this point, usage += PAGE_SIZE.
+
+        At commit(), the function checks the page should be charged or not
+        and set flags or avoid charging.(usage -= PAGE_SIZE)
+
+        At cancel(), simply usage -= PAGE_SIZE.
+
+Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
+
+4. Anonymous
+        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.
+
+        (a) If the SwapCache is newly allocated and read, it has no charges.
+        (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.
+
+        (a) add to swap cache. (marked as SwapCache)
+            swp_entry's refcnt += 1.
+        (b) fully unmapped.
+            swp_entry's refcnt += # of ptes.
+        (c) write back to swap.
+        (d) delete from swap cache. (remove from SwapCache)
+            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.
+        But brief explanation of the behavior of memcg around shmem will be
+        helpful to understand the logic.
+
+        Shmem's page (just leaf page, not direct/indirect block) can be on
+                - radix-tree of shmem's inode.
+                - SwapCache.
+                - Both on radix-tree and SwapCache. This happens at swap-in
+                  and swap-out,
+
+        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().
+
+8. LRU
+        Each memcg has its own private LRU. Now, it's handling is under global
+        VM's control (means that it's handled under global zone->lru_lock).
+        Almost all routines around memcg's LRU is called by global LRU's
+        list management functions under zone->lru_lock().
+
+        A special function is mem_cgroup_isolate_pages(). This scans
+        memcg's private LRU and call __isolate_lru_page() to extract a page
+        from LRU.
+        (By __isolate_lru_page(), the page is removed from both of global and
+         private LRU.)
+
+
+9. Typical Tests.
+
+ Tests for racy cases.
+
+ 9.1 Small limit to memcg.
+        When you do test to do racy case, it's good test to set memcg's limit
+        to be very small rather than GB. Many races found in the test under
+        xKB or xxMB limits.
+        (Memory behavior under GB and Memory behavior under MB shows very
+         different situation.)
+
+ 9.2 Shmem
+        Historically, memcg's shmem handling was poor and we saw some amount
+        of troubles here. This is because shmem is page-cache but can be
+        SwapCache. Test with shmem/tmpfs is always good test.
+
+ 9.3 Migration
+        For NUMA, migration is an another special case. To do easy test, cpuset
+        is useful. Following is a sample script to do migration.
+
+        mount -t cgroup -o cpuset none /opt/cpuset
+
+        mkdir /opt/cpuset/01
+        echo 1 > /opt/cpuset/01/cpuset.cpus
+        echo 0 > /opt/cpuset/01/cpuset.mems
+        echo 1 > /opt/cpuset/01/cpuset.memory_migrate
+        mkdir /opt/cpuset/02
+        echo 1 > /opt/cpuset/02/cpuset.cpus
+        echo 1 > /opt/cpuset/02/cpuset.mems
+        echo 1 > /opt/cpuset/02/cpuset.memory_migrate
+
+        In above set, when you moves a task from 01 to 02, page migration to
+        node 0 to node 1 will occur. Following is a script to migrate all
+        under cpuset.
+        --
+        move_task()
+        {
+        for pid in $1
+        do
+                /bin/echo $pid >$2/tasks 2>/dev/null
+                echo -n $pid
+                echo -n " "
+        done
+        echo END
+        }
+
+        G1_TASK=`cat ${G1}/tasks`
+        G2_TASK=`cat ${G2}/tasks`
+        move_task "${G1_TASK}" ${G2} &
+        --
+ 9.4 Memory hotplug.
+        memory hotplug test is one of good test.
+        to offline memory, do following.
+        # echo offline > /sys/devices/system/memory/memoryXXX/state
+        (XXX is the place of memory)
+        This is an easy way to test page migration, too.
+
+ 9.5 mkdir/rmdir
+        When using hierarchy, mkdir/rmdir test should be done.
+        Use tests like the following.
+
+        echo 1 >/opt/cgroup/01/memory/use_hierarchy
+        mkdir /opt/cgroup/01/child_a
+        mkdir /opt/cgroup/01/child_b
+
+        set limit to 01.
+        add limit to 01/child_b
+        run jobs under child_a and child_b
+
+        create/delete following groups at random while jobs are running.
+        /opt/cgroup/01/child_a/child_aa
+        /opt/cgroup/01/child_b/child_bb
+        /opt/cgroup/01/child_c
+
+        running new jobs in new group is also good.
+
+ 9.6 Mount with other subsystems.
+        Mounting with other subsystems is a good test because there is a
+        race and lock dependency with other cgroup subsystems.
+
+        example)
+        # mount -t cgroup none /cgroup -t cpuset,memory,cpu,devices
+
+        and do task move, mkdir, rmdir etc...under this.