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")
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.
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)
