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

[Usama Arif <usamaarif642@gmail.com>]

On 02/11/2024 10:12, 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")
I don't think its a hotfix (or even a fix). It was discussed in the initial
series to add these as a follow up and Joshua was going to do this soon.
Its not fixing any bug in the initial series.

> 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.
> 
I dont think either of them would be a hotfix.

>  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.
> +

Maybe zero-filled pages might be a better term in both. 

>  	  zswpin
>  		Number of pages moved in to memory from zswap.
>  
> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
> index aed952d04132..f70d0958095c 100644
> --- a/include/linux/vm_event_item.h
> +++ b/include/linux/vm_event_item.h
> @@ -134,6 +134,8 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>  #ifdef CONFIG_SWAP
>  		SWAP_RA,
>  		SWAP_RA_HIT,
> +		SWPIN_ZERO,
> +		SWPOUT_ZERO,
>  #ifdef CONFIG_KSM
>  		KSM_SWPIN_COPY,
>  #endif
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5e44d6e7591e..7b3503d12aaf 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -441,6 +441,10 @@ static const unsigned int memcg_vm_event_stat[] = {
>  	PGDEACTIVATE,
>  	PGLAZYFREE,
>  	PGLAZYFREED,
> +#ifdef CONFIG_SWAP
> +	SWPIN_ZERO,
> +	SWPOUT_ZERO,
> +#endif
>  #ifdef CONFIG_ZSWAP
>  	ZSWPIN,
>  	ZSWPOUT,
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 5d9b6e6cf96c..4b4ea8e49cf6 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -204,7 +204,9 @@ static bool is_folio_zero_filled(struct folio *folio)
>  
>  static void swap_zeromap_folio_set(struct folio *folio)
>  {
> +	struct obj_cgroup *objcg = get_obj_cgroup_from_folio(folio);
>  	struct swap_info_struct *sis = swp_swap_info(folio->swap);
> +	int nr_pages = folio_nr_pages(folio);
>  	swp_entry_t entry;
>  	unsigned int i;
>  
> @@ -212,6 +214,12 @@ static void swap_zeromap_folio_set(struct folio *folio)
>  		entry = page_swap_entry(folio_page(folio, i));
>  		set_bit(swp_offset(entry), sis->zeromap);
>  	}
> +
> +	count_vm_events(SWPOUT_ZERO, nr_pages);
> +	if (objcg) {
> +		count_objcg_events(objcg, SWPOUT_ZERO, nr_pages);
> +		obj_cgroup_put(objcg);
> +	}
>  }
>  
>  static void swap_zeromap_folio_clear(struct folio *folio)
> @@ -507,6 +515,7 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
>  static bool swap_read_folio_zeromap(struct folio *folio)
>  {
>  	int nr_pages = folio_nr_pages(folio);
> +	struct obj_cgroup *objcg;
>  	bool is_zeromap;
>  
>  	/*
> @@ -521,6 +530,13 @@ static bool swap_read_folio_zeromap(struct folio *folio)
>  	if (!is_zeromap)
>  		return false;
>  
> +	objcg = get_obj_cgroup_from_folio(folio);
> +	count_vm_events(SWPIN_ZERO, nr_pages);
> +	if (objcg) {
> +		count_objcg_events(objcg, SWPIN_ZERO, nr_pages);
> +		obj_cgroup_put(objcg);
> +	}
> +
>  	folio_zero_range(folio, 0, folio_size(folio));
>  	folio_mark_uptodate(folio);
>  	return true;
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 22a294556b58..c8ef7352f9ed 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1418,6 +1418,8 @@ const char * const vmstat_text[] = {
>  #ifdef CONFIG_SWAP
>  	"swap_ra",
>  	"swap_ra_hit",
> +	"swpin_zero",
> +	"swpout_zero",
>  #ifdef CONFIG_KSM
>  	"ksm_swpin_copy",
>  #endif