[PATCH v2 3/7] mm, swap: avoid redundant swap device pinning

Kairui Song
2025-02-24T18:03:02Z

Currently __read_swap_cache_async() has get/put_swap_device() calls to
increase/decrease a swap device reference to prevent swapoff. While some
of its callers have already held the swap device reference, e.g in
do_swap_page() and shmem_swapin_folio() where __read_swap_cache_async()
will finally called. Now there are only two callers not holding a swap
device reference, so make them hold a reference instead. And drop the
get/put_swap_device calls in __read_swap_cache_async. This should reduce
the overhead for swap in during page fault slightly.

Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
---
mm/swap_state.c | 14 ++++++++------
mm/zswap.c | 6 ++++++
2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index a54b035d6a6c..50840a2887a5 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -426,17 +426,13 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
struct mempolicy *mpol, pgoff_t ilx, bool *new_page_allocated,
bool skip_if_exists)
{
- struct swap_info_struct *si;
+ struct swap_info_struct *si = swp_swap_info(entry);
struct folio *folio;
struct folio *new_folio = NULL;
struct folio *result = NULL;
void *shadow = NULL;

*new_page_allocated = false;
- si = get_swap_device(entry);
- if (!si)
- return NULL;
-
for (;;) {
int err;
/*
@@ -532,7 +528,6 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
put_swap_folio(new_folio, entry);
folio_unlock(new_folio);
put_and_return:
- put_swap_device(si);
if (!(*new_page_allocated) && new_folio)
folio_put(new_folio);
return result;
@@ -552,11 +547,16 @@ struct folio *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
struct vm_area_struct *vma, unsigned long addr,
struct swap_iocb **plug)
{
+ struct swap_info_struct *si;
bool page_allocated;
struct mempolicy *mpol;
pgoff_t ilx;
struct folio *folio;

+ si = get_swap_device(entry);
+ if (!si)
+ return NULL;
+
mpol = get_vma_policy(vma, addr, 0, &ilx);
folio = __read_swap_cache_async(entry, gfp_mask, mpol, ilx,
&page_allocated, false);
@@ -564,6 +564,8 @@ struct folio *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,

if (page_allocated)
swap_read_folio(folio, plug);
+
+ put_swap_device(si);
return folio;
}

diff --git a/mm/zswap.c b/mm/zswap.c
index ac9d299e7d0c..83dfa1f9e689 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1051,14 +1051,20 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
struct folio *folio;
struct mempolicy *mpol;
bool folio_was_allocated;
+ struct swap_info_struct *si;
struct writeback_control wbc = {
.sync_mode = WB_SYNC_NONE,
};

/* try to allocate swap cache folio */
+ si = get_swap_device(swpentry);
+ if (!si)
+ return -EEXIST;
+
mpol = get_task_policy(current);
folio = __read_swap_cache_async(swpentry, GFP_KERNEL, mpol,
NO_INTERLEAVE_INDEX, &folio_was_allocated, true);
+ put_swap_device(si);
if (!folio)
return -ENOMEM;

--
2.48.1