Re: [PATCH] mm: swap: mTHP frees entries as a whole

[zhiguojiang <justinjiang@vivo.com>]

在 2024/8/6 10:07, Barry Song 写道:
> On Tue, Aug 6, 2024 at 2:01 PM zhiguojiang <justinjiang@vivo.com> wrote:
>>
>>
>> 在 2024/8/6 6:09, Barry Song 写道:
>>> On Tue, Aug 6, 2024 at 4:08 AM Zhiguo Jiang <justinjiang@vivo.com> wrote:
>>>> Support mTHP's attempt to free swap entries as a whole, which can avoid
>>>> frequent swap_info locking for every individual entry in
>>>> swapcache_free_entries(). When the swap_map count values corresponding
>>>> to all contiguous entries are all zero excluding SWAP_HAS_CACHE, the
>>>> entries will be freed directly by skippping percpu swp_slots caches.
>>>>
>>> No, this isn't quite good. Please review the work done by Chris and Kairui[1];
>>> they have handled it better. On a different note, I have a patch that can
>>> handle zap_pte_range() for swap entries in batches[2][3].
>> I'm glad to see your optimized submission about batch freeing swap
>> entries for
>> zap_pte_range(), sorry, I didn't see it before. My this patch can be
>> ignored.
> no worries, please help test and review the formal patch I sent:
> https://lore.kernel.org/linux-mm/20240806012409.61962-1-21cnbao@gmail.com/
I believe it's ok and valuable.  Looking forward to being merged soon.
>
> Please note that I didn't use a bitmap to avoid a large stack, and
> there is a real possibility of the below can occur, your patch can
> crash if the below is true:
> nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER
>
> Additionally, I quickly skip the case where
> swap_count(data_race(si->swap_map[start_offset]) != 1) to avoid regressions
> in cases that can't be batched.
>
>> Thanks
>> Zhiguo
>>
>>> [1] https://lore.kernel.org/linux-mm/20240730-swap-allocator-v5-5-cb9c148b9297@kernel.org/
>>> [2] https://lore.kernel.org/linux-mm/20240803091118.84274-1-21cnbao@gmail.com/
>>> [3] https://lore.kernel.org/linux-mm/CAGsJ_4wPnQqKOHx6iQcwO8bQzoBXKr2qY2AgSxMwTQCj3-8YWw@mail.gmail.com/
>>>
>>>> Signed-off-by: Zhiguo Jiang <justinjiang@vivo.com>
>>>> ---
>>>>    mm/swapfile.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++
>>>>    1 file changed, 61 insertions(+)
>>>>
>>>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>>>> index ea023fc25d08..829fb4cfb6ec
>>>> --- a/mm/swapfile.c
>>>> +++ b/mm/swapfile.c
>>>> @@ -1493,6 +1493,58 @@ static void swap_entry_range_free(struct swap_info_struct *p, swp_entry_t entry,
>>>>           swap_range_free(p, offset, nr_pages);
>>>>    }
>>>>
>>>> +/*
>>>> + * Free the contiguous swap entries as a whole, caller have to
>>>> + * ensure all entries belong to the same folio.
>>>> + */
>>>> +static void swap_entry_range_check_and_free(struct swap_info_struct *p,
>>>> +                                 swp_entry_t entry, int nr, bool *any_only_cache)
>>>> +{
>>>> +       const unsigned long start_offset = swp_offset(entry);
>>>> +       const unsigned long end_offset = start_offset + nr;
>>>> +       unsigned long offset;
>>>> +       DECLARE_BITMAP(to_free, SWAPFILE_CLUSTER) = { 0 };
>>>> +       struct swap_cluster_info *ci;
>>>> +       int i = 0, nr_setbits = 0;
>>>> +       unsigned char count;
>>>> +
>>>> +       /*
>>>> +        * Free and check swap_map count values corresponding to all contiguous
>>>> +        * entries in the whole folio range.
>>>> +        */
>>>> +       WARN_ON_ONCE(nr > SWAPFILE_CLUSTER);
>>>> +       ci = lock_cluster_or_swap_info(p, start_offset);
>>>> +       for (offset = start_offset; offset < end_offset; offset++, i++) {
>>>> +               if (data_race(p->swap_map[offset])) {
>>>> +                       count = __swap_entry_free_locked(p, offset, 1);
>>>> +                       if (!count) {
>>>> +                               bitmap_set(to_free, i, 1);
>>>> +                               nr_setbits++;
>>>> +                       } else if (count == SWAP_HAS_CACHE) {
>>>> +                               *any_only_cache = true;
>>>> +                       }
>>>> +               } else {
>>>> +                       WARN_ON_ONCE(1);
>>>> +               }
>>>> +       }
>>>> +       unlock_cluster_or_swap_info(p, ci);
>>>> +
>>>> +       /*
>>>> +        * If the swap_map count values corresponding to all contiguous entries are
>>>> +        * all zero excluding SWAP_HAS_CACHE, the entries will be freed directly by
>>>> +        * skippping percpu swp_slots caches, which can avoid frequent swap_info
>>>> +        * locking for every individual entry.
>>>> +        */
>>>> +       if (nr > 1 && nr_setbits == nr) {
>>>> +               spin_lock(&p->lock);
>>>> +               swap_entry_range_free(p, entry, nr);
>>>> +               spin_unlock(&p->lock);
>>>> +       } else {
>>>> +               for_each_set_bit(i, to_free, SWAPFILE_CLUSTER)
>>>> +                       free_swap_slot(swp_entry(p->type, start_offset + i));
>>>> +       }
>>>> +}
>>>> +
>>>>    static void cluster_swap_free_nr(struct swap_info_struct *sis,
>>>>                   unsigned long offset, int nr_pages,
>>>>                   unsigned char usage)
>>>> @@ -1808,6 +1860,14 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
>>>>           if (WARN_ON(end_offset > si->max))
>>>>                   goto out;
>>>>
>>>> +       /*
>>>> +        * Try to free all contiguous entries about mTHP as a whole.
>>>> +        */
>>>> +       if (IS_ENABLED(CONFIG_THP_SWAP) && nr > 1) {
>>>> +               swap_entry_range_check_and_free(si, entry, nr, &any_only_cache);
>>>> +               goto free_cache;
>>>> +       }
>>>> +
>>>>           /*
>>>>            * First free all entries in the range.
>>>>            */
>>>> @@ -1821,6 +1881,7 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
>>>>                   }
>>>>           }
>>>>
>>>> +free_cache:
>>>>           /*
>>>>            * Short-circuit the below loop if none of the entries had their
>>>>            * reference drop to zero.
>>>> --
>>>> 2.39.0
>>>>
> Thanks
> Barry
Thanks
Zhiguo