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 2. I'm curious if others have different
opinions, so I'm marking it as RFC.
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>
---
mm/page_io.c | 30 +++++++++++++++++-------------
1 file changed, 17 insertions(+), 13 deletions(-)
diff --git a/mm/page_io.c b/mm/page_io.c
index 5d9b6e6cf96c..90c5ea870038 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -226,6 +226,19 @@ static void swap_zeromap_folio_clear(struct folio *folio)
}
}
+static inline void count_swpout_vm_event(struct folio *folio)
+{
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+ if (unlikely(folio_test_pmd_mappable(folio))) {
+ count_memcg_folio_events(folio, THP_SWPOUT, 1);
+ count_vm_event(THP_SWPOUT);
+ }
+#endif
+ count_mthp_stat(folio_order(folio), MTHP_STAT_SWPOUT);
+ count_memcg_folio_events(folio, PSWPOUT, folio_nr_pages(folio));
+ count_vm_events(PSWPOUT, folio_nr_pages(folio));
+}
+
/*
* We may have stale swap cache pages in memory: notice
* them here and get rid of the unnecessary final write.
@@ -258,6 +271,7 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
*/
if (is_folio_zero_filled(folio)) {
swap_zeromap_folio_set(folio);
+ count_swpout_vm_event(folio);
folio_unlock(folio);
return 0;
} else {
@@ -282,19 +296,6 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
return 0;
}
-static inline void count_swpout_vm_event(struct folio *folio)
-{
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
- if (unlikely(folio_test_pmd_mappable(folio))) {
- count_memcg_folio_events(folio, THP_SWPOUT, 1);
- count_vm_event(THP_SWPOUT);
- }
-#endif
- count_mthp_stat(folio_order(folio), MTHP_STAT_SWPOUT);
- count_memcg_folio_events(folio, PSWPOUT, folio_nr_pages(folio));
- count_vm_events(PSWPOUT, folio_nr_pages(folio));
-}
-
#if defined(CONFIG_MEMCG) && defined(CONFIG_BLK_CGROUP)
static void bio_associate_blkg_from_page(struct bio *bio, struct folio *folio)
{
@@ -621,6 +622,9 @@ void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
delayacct_swapin_start();
if (swap_read_folio_zeromap(folio)) {
+ count_mthp_stat(folio_order(folio), MTHP_STAT_SWPIN);
+ count_memcg_folio_events(folio, PSWPIN, folio_nr_pages(folio));
+ count_vm_events(PSWPIN, folio_nr_pages(folio));
folio_unlock(folio);
goto finish;
} else if (zswap_load(folio)) {
--
2.39.3 (Apple Git-146)
