linux-kernel - Re: [PATCH 15/18] mm: memcontrol: make swap tracking an integral part of memory control

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Date:   Wed, 22 Apr 2020 09:30:01 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     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: Re: [PATCH 15/18] mm: memcontrol: make swap tracking an integral
 part of memory control

On Wed, Apr 22, 2020 at 11:14:40AM +0800, Alex Shi wrote:
> 
> 
> 在 2020/4/21 下午10:39, Johannes Weiner 写道:
> > Hi Alex,
> > 
> > thanks for your quick review so far, I'll add the tags to the patches.
> > 
> > On Tue, Apr 21, 2020 at 05:27:30PM +0800, Alex Shi wrote:
> >>
> >>
> >> 在 2020/4/21 上午6:11, Johannes Weiner 写道:
> >>> The swapaccount=0 boot option will continue to exist, and it will
> >>> eliminate the page_counter overhead and hide the swap control files,
> >>> but it won't disable swap slot ownership tracking.
> >>
> >> May we add extra explanation for this change to user? and the default
> >> memsw limitations?
> > 
> > Can you elaborate what you think is missing and where you would like
> > to see it documented?
> > 
> Maybe the following doc change is better after whole patchset? 
> Guess users would would happy to know details of this change.

Thanks, I stole your patch and extended/tweaked it a little. Would you
mind providing your Signed-off-by:?

>From 589d3c1b505e6671b4a9b424436c9eda88a0b08c Mon Sep 17 00:00:00 2001
From: Alex Shi <alex.shi@...ux.alibaba.com>
Date: Wed, 22 Apr 2020 11:14:40 +0800
Subject: [PATCH] mm: memcontrol: document the new swap control behavior

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.
-- 



> Also as to the RSS account name change, I don't know if it's good to polish
> them in docs.

I didn't actually change anything user-visible, just the internal name
of the counters:

static const unsigned int memcg1_stats[] = {
	NR_FILE_PAGES,		/* was MEMCG_CACHE */
	NR_ANON_MAPPED,		/* was MEMCG_RSS */
	NR_ANON_THPS,		/* was MEMCG_RSS_HUGE */
	NR_SHMEM,
	NR_FILE_MAPPED,
	NR_FILE_DIRTY,
	NR_WRITEBACK,
	MEMCG_SWAP,
};

static const char *const memcg1_stat_names[] = {
	"cache",
	"rss",
	"rss_huge",
	"shmem",
	"mapped_file",
	"dirty",
	"writeback",
	"swap",
};

Or did you refer to something else?