linux-kernel - Re: [PATCH v2 1/5] mm: swap: introduce swap_free_nr() for batched swap

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Message-ID: <66afc978-0221-488b-9fc6-7d5213d385ed@arm.com>
Date: Fri, 12 Apr 2024 12:28:23 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Chuanhua Han <chuanhuahan@...il.com>
Cc: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org, baolin.wang@...ux.alibaba.com, chrisl@...nel.org,
 david@...hat.com, hanchuanhua@...o.com, hannes@...xchg.org,
 hughd@...gle.com, kasong@...cent.com, surenb@...gle.com,
 v-songbaohua@...o.com, willy@...radead.org, xiang@...nel.org,
 ying.huang@...el.com, yosryahmed@...gle.com, yuzhao@...gle.com,
 ziy@...dia.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/5] mm: swap: introduce swap_free_nr() for batched
 swap_free()

On 12/04/2024 03:07, Chuanhua Han wrote:
> Ryan Roberts <ryan.roberts@....com> 于2024年4月11日周四 22:30写道：
>>
>> On 09/04/2024 09:26, Barry Song wrote:
>>> From: Chuanhua Han <hanchuanhua@...o.com>
>>>
>>> While swapping in a large folio, we need to free swaps related to the whole
>>> folio. To avoid frequently acquiring and releasing swap locks, it is better
>>> to introduce an API for batched free.
>>>
>>> Signed-off-by: Chuanhua Han <hanchuanhua@...o.com>
>>> Co-developed-by: Barry Song <v-songbaohua@...o.com>
>>> Signed-off-by: Barry Song <v-songbaohua@...o.com>
>>
>> Couple of nits; feel free to ignore.
>>
>> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
>>
>>> ---
>>>  include/linux/swap.h |  5 +++++
>>>  mm/swapfile.c        | 51 ++++++++++++++++++++++++++++++++++++++++++++
>>>  2 files changed, 56 insertions(+)
>>>
>>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>>> index 11c53692f65f..b7a107e983b8 100644
>>> --- a/include/linux/swap.h
>>> +++ b/include/linux/swap.h
>>> @@ -483,6 +483,7 @@ extern void swap_shmem_alloc(swp_entry_t);
>>>  extern int swap_duplicate(swp_entry_t);
>>>  extern int swapcache_prepare(swp_entry_t);
>>>  extern void swap_free(swp_entry_t);
>>> +extern void swap_free_nr(swp_entry_t entry, int nr_pages);
>>>  extern void swapcache_free_entries(swp_entry_t *entries, int n);
>>>  extern void free_swap_and_cache_nr(swp_entry_t entry, int nr);
>>>  int swap_type_of(dev_t device, sector_t offset);
>>> @@ -564,6 +565,10 @@ static inline void swap_free(swp_entry_t swp)
>>>  {
>>>  }
>>>
>>> +void swap_free_nr(swp_entry_t entry, int nr_pages)
>>> +{
>>> +}
>>> +
>>>  static inline void put_swap_folio(struct folio *folio, swp_entry_t swp)
>>>  {
>>>  }
>>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>>> index 28642c188c93..f4c65aeb088d 100644
>>> --- a/mm/swapfile.c
>>> +++ b/mm/swapfile.c
>>> @@ -1356,6 +1356,57 @@ void swap_free(swp_entry_t entry)
>>>               __swap_entry_free(p, entry);
>>>  }
>>>
>>> +/*
>>> + * Free up the maximum number of swap entries at once to limit the
>>> + * maximum kernel stack usage.
>>> + */
>>> +#define SWAP_BATCH_NR (SWAPFILE_CLUSTER > 512 ? 512 : SWAPFILE_CLUSTER)
>>> +
>>> +/*
>>> + * Called after swapping in a large folio, batched free swap entries
>>> + * for this large folio, entry should be for the first subpage and
>>> + * its offset is aligned with nr_pages
>>> + */
>>> +void swap_free_nr(swp_entry_t entry, int nr_pages)
>>> +{
>>> +     int i, j;
>>> +     struct swap_cluster_info *ci;
>>> +     struct swap_info_struct *p;
>>> +     unsigned int type = swp_type(entry);
>>> +     unsigned long offset = swp_offset(entry);
>>> +     int batch_nr, remain_nr;
>>> +     DECLARE_BITMAP(usage, SWAP_BATCH_NR) = { 0 };
>>> +
>>> +     /* all swap entries are within a cluster for mTHP */
>>> +     VM_BUG_ON(offset % SWAPFILE_CLUSTER + nr_pages > SWAPFILE_CLUSTER);
>>> +
>>> +     if (nr_pages == 1) {
>>> +             swap_free(entry);
>>> +             return;
>>> +     }
>>> +
>>> +     remain_nr = nr_pages;
>>> +     p = _swap_info_get(entry);
>>> +     if (p) {
>>
>> nit: perhaps return early if (!p) ? Then you dedent the for() block.
> 
> Agreed!
> 
>>
>>> +             for (i = 0; i < nr_pages; i += batch_nr) {
>>> +                     batch_nr = min_t(int, SWAP_BATCH_NR, remain_nr);
>>> +
>>> +                     ci = lock_cluster_or_swap_info(p, offset);
>>> +                     for (j = 0; j < batch_nr; j++) {
>>> +                             if (__swap_entry_free_locked(p, offset + i * SWAP_BATCH_NR + j, 1))
>>> +                                     __bitmap_set(usage, j, 1);
>>> +                     }
>>> +                     unlock_cluster_or_swap_info(p, ci);
>>> +
>>> +                     for_each_clear_bit(j, usage, batch_nr)
>>> +                             free_swap_slot(swp_entry(type, offset + i * SWAP_BATCH_NR + j));
>>> +
>>
>> nit: perhaps change to for (;;), and do the checks here to avoid clearing the
>> bitmap on the last run:
>>
>>                         i += batch_nr;
>>                         if (i < nr_pages)
>>                                 break;
> Great, thank you for your advice!

Or maybe leave the for() as is, but don't explicitly init the bitmap at the
start of the function and instead call:

	bitmap_clear(usage, 0, SWAP_BATCH_NR);

At the start of each loop?

>>
>>> +                     bitmap_clear(usage, 0, SWAP_BATCH_NR);
>>> +                     remain_nr -= batch_nr;
>>> +             }
>>> +     }
>>> +}
>>> +
>>>  /*
>>>   * Called after dropping swapcache to decrease refcnt to swap entries.
>>>   */
>>
>>
> 
>