linux-kernel - [PATCH 17/19] mm: memcontrol: document the new swap control behavior

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Date:   Fri,  8 May 2020 14:31:04 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Alex Shi <alex.shi@...ux.alibaba.com>,
        Joonsoo Kim <js1304@...il.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Roman Gushchin <guro@...com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: [PATCH 17/19] mm: memcontrol: document the new swap control behavior

From: Alex Shi <alex.shi@...ux.alibaba.com>

Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com>
Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
 .../admin-guide/cgroup-v1/memory.rst          | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
index 0ae4f564c2d6..12757e63b26c 100644
--- a/Documentation/admin-guide/cgroup-v1/memory.rst
+++ b/Documentation/admin-guide/cgroup-v1/memory.rst
@@ -199,11 +199,11 @@ An RSS page is unaccounted when it's fully unmapped. A PageCache page is
 unaccounted when it's removed from radix-tree. Even if RSS pages are fully
 unmapped (by kswapd), they may exist as SwapCache in the system until they
 are really freed. Such SwapCaches are also accounted.
-A swapped-in page is not accounted until it's mapped.
+A swapped-in page is accounted after adding into swapcache.
 
 Note: The kernel does swapin-readahead and reads multiple swaps at once.
-This means swapped-in pages may contain pages for other tasks than a task
-causing page fault. So, we avoid accounting at swap-in I/O.
+Since page's memcg recorded into swap whatever memsw enabled, the page will
+be accounted after swapin.
 
 At page migration, accounting information is kept.
 
@@ -222,18 +222,13 @@ the cgroup that brought it in -- this will happen on memory pressure).
 But see section 8.2: when moving a task to another cgroup, its pages may
 be recharged to the new cgroup, if move_charge_at_immigrate has been chosen.
 
-Exception: If CONFIG_MEMCG_SWAP is not used.
-When you do swapoff and make swapped-out pages of shmem(tmpfs) to
-be backed into memory in force, charges for pages are accounted against the
-caller of swapoff rather than the users of shmem.
-
-2.4 Swap Extension (CONFIG_MEMCG_SWAP)
+2.4 Swap Extension
 --------------------------------------
 
-Swap Extension allows you to record charge for swap. A swapped-in page is
-charged back to original page allocator if possible.
+Swap usage is always recorded for each of cgroup. Swap Extension allows you to
+read and limit it.
 
-When swap is accounted, following files are added.
+When CONFIG_SWAP is enabled, following files are added.
 
  - memory.memsw.usage_in_bytes.
  - memory.memsw.limit_in_bytes.
-- 
2.26.2