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

Johannes Weiner
2020-04-22T13:30:07Z

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@linux.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@cmpxchg.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?