Re: [PATCH v2 4/4] mm: zswap: skip zswap_invalidate() in swap_range_free() when zswap is unused

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 9/10/2026 6:29 PM, Yosry Ahmed wrote:
> On Thu, Sep 10, 2026 at 2:55 AM Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
>>
>> swap_range_free() calls zswap_invalidate() for every slot being
>> freed, even when zswap has never been enabled. Guard the loop with
>> zswap_never_enabled() to skip it.
>>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>>   mm/swapfile.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index ba71905e4d46..505592051924 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -1318,8 +1318,10 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
>>          void (*swap_slot_free_notify)(struct block_device *, unsigned long);
>>          unsigned int i;
>>
>> -       for (i = 0; i < nr_entries; i++)
>> -               zswap_invalidate(si->type, offset + i);
>> +       if (!zswap_never_enabled()) {
>> +               for (i = 0; i < nr_entries; i++)
>> +                       zswap_invalidate(si->type, offset + i);
>> +       }
> 
> What if we add the check in zswap_invalidate()? I understand we'd
> avoid the loop here, which is nice, but I wonder if it's actually a
> measurable difference.
> 

Skipping useless instructions is always a good thing.

> If we keep it in zswap_invalidate(), we can probably also skip patch 3?

Maybe add nr_entries to zswap_invalidate() and check
zswap_never_enabled() in it.

diff --git a/include/linux/zswap.h b/include/linux/zswap.h
index 463bdee5c1e1..313c2f1b6c2e 100644
--- a/include/linux/zswap.h
+++ b/include/linux/zswap.h
@@ -27,7 +27,7 @@ struct zswap_lruvec_state {
  unsigned long zswap_total_pages(void);
  bool zswap_store(struct folio *folio);
  int zswap_load(struct folio *folio);
-void zswap_invalidate(int type, pgoff_t offset);
+void zswap_invalidate(int type, pgoff_t offset, unsigned int nr_entries);
  int zswap_swapon(int type, unsigned long nr_pages);
  void zswap_swapoff(int type);
  void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg);
@@ -49,7 +49,7 @@ static inline int zswap_load(struct folio *folio)
         return -ENOENT;
  }

-static inline void zswap_invalidate(int type, pgoff_t offset) {}
+static inline void zswap_invalidate(int type, pgoff_t offset, unsigned 
int nr_entries) {}
  static inline int zswap_swapon(int type, unsigned long nr_pages)
  {
         return 0;
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 505592051924..891379c95a01 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1316,12 +1316,8 @@ static void swap_range_free(struct 
swap_info_struct *si, unsigned long offset,
  {
         unsigned long end = offset + nr_entries - 1;
         void (*swap_slot_free_notify)(struct block_device *, unsigned 
long);
-       unsigned int i;

-       if (!zswap_never_enabled()) {
-               for (i = 0; i < nr_entries; i++)
-                       zswap_invalidate(si->type, offset + i);
-       }
+       zswap_invalidate(si->type, offset, nr_entries);

         if (si->flags & SWP_BLKDEV)
                 swap_slot_free_notify =
diff --git a/mm/zswap.c b/mm/zswap.c
index 6197aa71e33c..0e72cc9a5415 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1549,13 +1549,8 @@ bool zswap_store(struct folio *folio)
          * offsets corresponding to each page of the folio. Otherwise,
          * writeback could overwrite the new data in the swapfile.
          */
-       if (!ret) {
-               unsigned type = swp_type(swp);
-               pgoff_t offset = swp_offset(swp);
-
-               for (index = 0; index < nr_pages; ++index)
-                       zswap_invalidate(type, offset + index);
-       }
+       if (!ret)
+               zswap_invalidate(swp_type(swp), swp_offset(swp), nr_pages);

         return ret;
  }
@@ -1661,17 +1656,25 @@ int zswap_load(struct folio *folio)
         return 0;
  }

-void zswap_invalidate(int type, pgoff_t offset)
+void zswap_invalidate(int type, pgoff_t offset, int nr_entries)
  {
-       struct xarray *tree = zswap_tree(type, offset);
         struct zswap_entry *entry;
+       struct xarray *tree;
+       int i;

-       if (xa_empty(tree))
+       if (!zswap_never_enabled())
                 return;

-       entry = xa_erase(tree, offset);
-       if (entry)
-               zswap_entry_free(entry);
+       for (i = 0; i < nr_entries; ++i) {
+               tree = zswap_tree(type, offset + i);
+
+               if (xa_empty(tree))
+                       return;
+
+               entry = xa_erase(tree, offset + i);
+               if (entry)
+                       zswap_entry_free(entry);
+       }
  }

If no objections, I will refresh all the patches.