linux-kernel - Re: [PATCH v2] mm: count zeromap read and set for swapout and swapin

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Message-ID: <c7a90ccf-c1b1-480c-9f2a-88ef37c3d89e@redhat.com>
Date: Mon, 4 Nov 2024 13:42:08 +0100
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>,
 Usama Arif <usamaarif642@...il.com>,
 Chengming Zhou <chengming.zhou@...ux.dev>,
 Yosry Ahmed <yosryahmed@...gle.com>, Nhat Pham <nphamcs@...il.com>,
 Johannes Weiner <hannes@...xchg.org>, Hugh Dickins <hughd@...gle.com>,
 Matthew Wilcox <willy@...radead.org>, Shakeel Butt <shakeel.butt@...ux.dev>,
 Andi Kleen <ak@...ux.intel.com>, Baolin Wang
 <baolin.wang@...ux.alibaba.com>, Chris Li <chrisl@...nel.org>,
 "Huang, Ying" <ying.huang@...el.com>, Kairui Song <kasong@...cent.com>,
 Ryan Roberts <ryan.roberts@....com>
Subject: Re: [PATCH v2] mm: count zeromap read and set for swapout and swapin

On 02.11.24 11:12, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
> 
> When the proportion of folios from the zero map is small, missing their
> accounting may not significantly impact profiling. However, it’s easy
> to construct a scenario where this becomes an issue—for example,
> allocating 1 GB of memory, writing zeros from userspace, followed by
> MADV_PAGEOUT, and then swapping it back in. In this case, the swap-out
> and swap-in counts seem to vanish into a black hole, potentially
> causing semantic ambiguity.
> 
> We have two ways to address this:
> 
> 1. Add a separate counter specifically for the zero map.
> 2. Continue using the current accounting, treating the zero map like
> a normal backend. (This aligns with the current behavior of zRAM
> when supporting same-page fills at the device level.)
> 
> This patch adopts option 1 as pswpin/pswpout counters are that they
> only apply to IO done directly to the backend device (as noted by
> Nhat Pham).
> 
> We can find these counters from /proc/vmstat (counters for the whole
> system) and memcg's memory.stat (counters for the interested memcg).
> 
> For example:
> 
> $ grep -E 'swpin_zero|swpout_zero' /proc/vmstat
> swpin_zero 1648
> swpout_zero 33536
> 
> $ grep -E 'swpin_zero|swpout_zero' /sys/fs/cgroup/system.slice/memory.stat
> swpin_zero 3905
> swpout_zero 3985
> 
> Fixes: 0ca0c24e3211 ("mm: store zero pages to be swapped out in a bitmap")
> Cc: Usama Arif <usamaarif642@...il.com>
> Cc: Chengming Zhou <chengming.zhou@...ux.dev>
> Cc: Yosry Ahmed <yosryahmed@...gle.com>
> Cc: Nhat Pham <nphamcs@...il.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
> Cc: Shakeel Butt <shakeel.butt@...ux.dev>
> Cc: Andi Kleen <ak@...ux.intel.com>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: Chris Li <chrisl@...nel.org>
> Cc: "Huang, Ying" <ying.huang@...el.com>
> Cc: Kairui Song <kasong@...cent.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
>   -v2:
>   * add separate counters rather than using pswpin/out; thanks
>   for the comments from Usama, David, Yosry and Nhat;
>   * Usama also suggested a new counter like swapped_zero, I
>   prefer that one be separated as an enhancement patch not
>   a hotfix. will probably handle it later on.
> 
>   Documentation/admin-guide/cgroup-v2.rst | 10 ++++++++++
>   include/linux/vm_event_item.h           |  2 ++
>   mm/memcontrol.c                         |  4 ++++
>   mm/page_io.c                            | 16 ++++++++++++++++
>   mm/vmstat.c                             |  2 ++
>   5 files changed, 34 insertions(+)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index db3799f1483e..984eb3c9d05b 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1599,6 +1599,16 @@ The following nested keys are defined.
>   	  pglazyfreed (npn)
>   		Amount of reclaimed lazyfree pages
>   
> +	  swpin_zero
> +		Number of pages moved into memory with zero content, meaning no
> +		copy exists in the backend swapfile, allowing swap-in to avoid
> +		I/O read overhead.
> +
> +	  swpout_zero
> +		Number of pages moved out of memory with zero content, meaning no
> +		copy is needed in the backend swapfile, allowing swap-out to avoid
> +		I/O write overhead.

Hm, can make it a bit clearer that this is a pure optimization and refer 
to the other counters?

swpin_zero
	Portion of "pswpin" pages for which I/O was optimized out
	because the page content was detected to be zero during swapout.

swpout_zero
	Portion of "pswout" pages for which I/O was optimized out
	because the page content was detected to be zero.


-- 
Cheers,

David / dhildenb