[RFC PATCH] mm: bypass swap readahead for zswap

[Alexandre Ghiti]

Commit 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous
device") made SWP_SYNCHRONOUS_IO devices (e.g. zram) skip swap readahead.

zswap is the same kind of in-memory, synchronous backend as zram, not a
swap device flagged SWP_SYNCHRONOUS_IO so it still goes through
swapin_readahead().

Here are the results from bypassing readahead for zswap too: it was
measured with a kernel build (make -j16) in a memcg, zswap=zstd, shrinker
off, on Sapphire Rapids and 3 iterations.

  768M memcg (sustained swap thrash):
    metric                 mm-new    + bypass    delta
    build time (s)          405.0       341.7    -15.6%
    zswap-in (GB)            79.5        53.0     -33%
    zswap-out (GB)          144.8       115.6     -20%
    swap readahead (pages)  6.79M       0.45M     -93%
    swap_ra hit (%)          72.1        89.9     +18pp

  1G memcg (light pressure, build not memory-bound):
    metric                 mm-new    + bypass    delta
    build time (s)          177.7       176.0    ~same (no regression)
    zswap-in (GB)            10.2         7.5     -26%
    zswap-out (GB)           27.7        25.1      -9%
    swap readahead (pages)  1.07M       0.08M     -93%
    swap_ra hit (%)          68.6        87.2     +19pp

The gain is from no longer prefetching pages that are pointless for an
in-memory backend: readahead inflates anon residency and thrashes the
page cache (file pages get evicted and re-read), lengthens each fault by
synchronously (de)compressing a cluster of neighbours, and adds
compression traffic when those extra pages are reclaimed.

Bypassing swap readahead for zswap therefore makes sense.

Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
---

- This bypass originally comes from Usama's series that implements
  large folio zswapin: while working on improving this series, I noticed
  the gains I got only came from the bypass of readahead.

 include/linux/zswap.h |  6 ++++++
 mm/memory.c           |  5 +++--
 mm/zswap.c            | 11 +++++++++++
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/include/linux/zswap.h b/include/linux/zswap.h
index 30c193a1207e..b6f0e6198b6f 100644
--- a/include/linux/zswap.h
+++ b/include/linux/zswap.h
@@ -35,6 +35,7 @@ void zswap_lruvec_state_init(struct lruvec *lruvec);
 void zswap_folio_swapin(struct folio *folio);
 bool zswap_is_enabled(void);
 bool zswap_never_enabled(void);
+bool zswap_present_test(swp_entry_t swp);
 #else
 
 struct zswap_lruvec_state {};
@@ -69,6 +70,11 @@ static inline bool zswap_never_enabled(void)
 	return true;
 }
 
+static inline bool zswap_present_test(swp_entry_t swp)
+{
+	return false;
+}
+
 #endif
 
 #endif /* _LINUX_ZSWAP_H */
diff --git a/mm/memory.c b/mm/memory.c
index ff338c2abe92..5aa1ea9eb48a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4827,8 +4827,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	if (folio)
 		swap_update_readahead(folio, vma, vmf->address);
 	if (!folio) {
-		/* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices */
-		if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
+		/* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices and zswap */
+		if (data_race(si->flags & SWP_SYNCHRONOUS_IO) ||
+		    zswap_present_test(entry))
 			folio = swapin_sync(entry, GFP_HIGHUSER_MOVABLE,
 					    thp_swapin_suitable_orders(vmf) | BIT(0),
 					    vmf, NULL, 0);
diff --git a/mm/zswap.c b/mm/zswap.c
index 761cd699e0a3..5b85b4d17647 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -234,6 +234,17 @@ static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
 		>> ZSWAP_ADDRESS_SPACE_SHIFT];
 }
 
+/**
+ * zswap_present_test - check if a swap entry is currently backed by zswap
+ * @swp: the swap entry to test
+ *
+ * Return: true if @swp has a zswap entry, false otherwise.
+ */
+bool zswap_present_test(swp_entry_t swp)
+{
+	return xa_load(swap_zswap_tree(swp), swp_offset(swp));
+}
+
 #define zswap_pool_debug(msg, p)			\
 	pr_debug("%s pool %s\n", msg, (p)->tfm_name)
 
-- 
2.54.0