On 02/11/2024 10:12, Barry Song wrote:
> From: Barry Song <v-songbaohua@xxxxxxxx>
>
> 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@xxxxxxxxx>
> Cc: Chengming Zhou <chengming.zhou@xxxxxxxxx>
> Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Cc: Nhat Pham <nphamcs@xxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: Chris Li <chrisl@xxxxxxxxxx>
> Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Kairui Song <kasong@xxxxxxxxxxx>
> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> ---
> -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
