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

[Barry Song <21cnbao@gmail.com>]

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.
+
 	  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
-- 
2.39.3 (Apple Git-146)