Re: [PATCH v2 2/8] mm: swap: remove unneeded VM_BUG_ON(*map != SWAP_HAS_CACHE) in swap_entry_range_free()

[Kemeng Shi <shikemeng@huaweicloud.com>]

on 3/19/2025 1:18 PM, Kairui Song wrote:
> On Tue, Mar 18, 2025 at 2:10 PM Kemeng Shi <shikemeng@huaweicloud.com> wrote:
>>
>> As all callers of swap_entry_range_free() have already ensured slots to
>> be freed are marked as SWAP_HAS_CACHE while holding the cluster lock,
>> the BUG_ON check can be safely removed. After this, the function
>> swap_entry_range_free() could drop any kind of last flag, rename it to
>> swap_entries_free() and update it's comment accordingly.
>>
>> This is a preparation to use swap_entries_free() to drop last ref count
>> and SWAP_MAP_SHMEM flag.
>>
>> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
>> Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
>> ---
>>  mm/swapfile.c | 27 +++++++++++++--------------
>>  1 file changed, 13 insertions(+), 14 deletions(-)
>>
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 5a775456e26c..0aa7ce82c013 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -52,9 +52,9 @@
>>  static bool swap_count_continued(struct swap_info_struct *, pgoff_t,
>>                                  unsigned char);
>>  static void free_swap_count_continuations(struct swap_info_struct *);
>> -static void swap_entry_range_free(struct swap_info_struct *si,
>> -                                 struct swap_cluster_info *ci,
>> -                                 swp_entry_t entry, unsigned int nr_pages);
>> +static void swap_entries_free(struct swap_info_struct *si,
>> +                             struct swap_cluster_info *ci,
>> +                             swp_entry_t entry, unsigned int nr_pages);
>>  static void swap_range_alloc(struct swap_info_struct *si,
>>                              unsigned int nr_entries);
>>  static bool folio_swapcache_freeable(struct folio *folio);
>> @@ -1463,7 +1463,7 @@ static unsigned char swap_entry_put(struct swap_info_struct *si,
>>         ci = lock_cluster(si, offset);
>>         usage = swap_entry_put_locked(si, offset, 1);
>>         if (!usage)
>> -               swap_entry_range_free(si, ci, swp_entry(si->type, offset), 1);
>> +               swap_entries_free(si, ci, swp_entry(si->type, offset), 1);
>>         unlock_cluster(ci);
>>
>>         return usage;
>> @@ -1493,7 +1493,7 @@ static bool swap_entries_put_nr(struct swap_info_struct *si,
>>         for (i = 0; i < nr; i++)
>>                 WRITE_ONCE(si->swap_map[offset + i], SWAP_HAS_CACHE);
>>         if (!has_cache)
>> -               swap_entry_range_free(si, ci, entry, nr);
>> +               swap_entries_free(si, ci, entry, nr);
>>         unlock_cluster(ci);
>>
>>         return has_cache;
>> @@ -1512,12 +1512,12 @@ static bool swap_entries_put_nr(struct swap_info_struct *si,
>>  }
>>
>>  /*
>> - * Drop the last HAS_CACHE flag of swap entries, caller have to
>> - * ensure all entries belong to the same cgroup.
>> + * Drop the last flag(1, SWAP_HAS_CACHE or SWAP_MAP_SHMEM) of swap entries,
>> + * caller have to ensure all entries belong to the same cgroup and cluster.
>>   */
>> -static void swap_entry_range_free(struct swap_info_struct *si,
>> -                                 struct swap_cluster_info *ci,
>> -                                 swp_entry_t entry, unsigned int nr_pages)
>> +static void swap_entries_free(struct swap_info_struct *si,
>> +                             struct swap_cluster_info *ci,
>> +                             swp_entry_t entry, unsigned int nr_pages)
>>  {
>>         unsigned long offset = swp_offset(entry);
>>         unsigned char *map = si->swap_map + offset;
>> @@ -1530,7 +1530,6 @@ static void swap_entry_range_free(struct swap_info_struct *si,
>>
>>         ci->count -= nr_pages;
>>         do {
>> -               VM_BUG_ON(*map != SWAP_HAS_CACHE);
> 
> Hi Kemeng
> 
> Instead of just dropping this check, maybe it's better to change it to
> something like VM_BUG_ON(!*map); to catch potential SWAP double free?
> I've found this check very helpful for debugging, especially for
> catching concurrency problems.
Sure, will keep VM_BUG_ON as it's useful.
> 
> Or more strictly: VM_BUG_ON(*map !=  SWAP_HAS_CACHE && *map != 1 &&
> *map != SWAP_MAP_SHMEM);, you may introduce a helper to check if a
> entry is the "last map" like this and use it somewhere else too.
I'd add VM_BUG_ON in this way to catch unexpected freeing.

Thanks,
Kemeng
> 
> 
>>                 *map = 0;
>>         } while (++map < map_end);
>>
>> @@ -1553,7 +1552,7 @@ static void cluster_swap_free_nr(struct swap_info_struct *si,
>>         ci = lock_cluster(si, offset);
>>         do {
>>                 if (!swap_entry_put_locked(si, offset, usage))
>> -                       swap_entry_range_free(si, ci, swp_entry(si->type, offset), 1);
>> +                       swap_entries_free(si, ci, swp_entry(si->type, offset), 1);
>>         } while (++offset < end);
>>         unlock_cluster(ci);
>>  }
>> @@ -1596,11 +1595,11 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
>>
>>         ci = lock_cluster(si, offset);
>>         if (swap_only_has_cache(si, offset, size))
>> -               swap_entry_range_free(si, ci, entry, size);
>> +               swap_entries_free(si, ci, entry, size);
>>         else {
>>                 for (int i = 0; i < size; i++, entry.val++) {
>>                         if (!swap_entry_put_locked(si, offset + i, SWAP_HAS_CACHE))
>> -                               swap_entry_range_free(si, ci, entry, 1);
>> +                               swap_entries_free(si, ci, entry, 1);
>>                 }
>>         }
>>         unlock_cluster(ci);
>> --
>> 2.30.0
>>
>