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

[David Hildenbrand <david@redhat.com>]

On 02.11.24 11:12, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.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@gmail.com>
> Cc: Chengming Zhou <chengming.zhou@linux.dev>
> Cc: Yosry Ahmed <yosryahmed@google.com>
> Cc: Nhat Pham <nphamcs@gmail.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> Cc: Shakeel Butt <shakeel.butt@linux.dev>
> Cc: Andi Kleen <ak@linux.intel.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Chris Li <chrisl@kernel.org>
> Cc: "Huang, Ying" <ying.huang@intel.com>
> Cc: Kairui Song <kasong@tencent.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Signed-off-by: Barry Song <v-songbaohua@oppo.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