Re: [PATCH v3 1/2] mm: store zero pages to be swapped out in a bitmap

[Usama Arif <usamaarif642@gmail.com>]

On 11/06/2024 19:39, Nhat Pham wrote:
> On Mon, Jun 10, 2024 at 5:18 AM Usama Arif <usamaarif642@gmail.com> wrote:
>> Approximately 10-20% of pages to be swapped out are zero pages [1].
>> Rather than reading/writing these pages to flash resulting
>> in increased I/O and flash wear, a bitmap can be used to mark these
>> pages as zero at write time, and the pages can be filled at
>> read time if the bit corresponding to the page is set.
>> With this patch, NVMe writes in Meta server fleet decreased
>> by almost 10% with conventional swap setup (zswap disabled).
>>
>> [1]https://lore.kernel.org/all/20171018104832epcms5p1b2232e2236258de3d03d1344dde9fce0@epcms5p1/
>>
>> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
>> ---
>>   include/linux/swap.h |  1 +
>>   mm/page_io.c         | 92 +++++++++++++++++++++++++++++++++++++++++++-
>>   mm/swapfile.c        | 21 +++++++++-
>>   3 files changed, 111 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index a11c75e897ec..e88563978441 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -299,6 +299,7 @@ struct swap_info_struct {
>>          signed char     type;           /* strange name for an index */
>>          unsigned int    max;            /* extent of the swap_map */
>>          unsigned char *swap_map;        /* vmalloc'ed array of usage counts */
>> +       unsigned long *zeromap;         /* vmalloc'ed bitmap to track zero pages */
>>          struct swap_cluster_info *cluster_info; /* cluster info. Only for SSD */
>>          struct swap_cluster_list free_clusters; /* free clusters list */
>>          unsigned int lowest_bit;        /* index of first free in swap_map */
>> diff --git a/mm/page_io.c b/mm/page_io.c
>> index a360857cf75d..2cac1e11fb85 100644
>> --- a/mm/page_io.c
>> +++ b/mm/page_io.c
>> @@ -172,6 +172,82 @@ int generic_swapfile_activate(struct swap_info_struct *sis,
>>          goto out;
>>   }
>>
>> +static bool is_folio_page_zero_filled(struct folio *folio, int i)
>> +{
>> +       unsigned long *data;
>> +       unsigned int pos, last_pos = PAGE_SIZE / sizeof(*data) - 1;
>> +       bool ret = false;
>> +
>> +       data = kmap_local_folio(folio, i * PAGE_SIZE);
>> +       if (data[last_pos])
>> +               goto out;
>> +       for (pos = 0; pos < PAGE_SIZE / sizeof(*data); pos++) {
>> +               if (data[pos])
>> +                       goto out;
>> +       }
>> +       ret = true;
>> +out:
>> +       kunmap_local(data);
>> +       return ret;
>> +}
>> +
>> +static bool is_folio_zero_filled(struct folio *folio)
>> +{
>> +       unsigned int i;
>> +
>> +       for (i = 0; i < folio_nr_pages(folio); i++) {
>> +               if (!is_folio_page_zero_filled(folio, i))
>> +                       return false;
>> +       }
>> +       return true;
>> +}
>> +
>> +static void folio_zero_fill(struct folio *folio)
>> +{
>> +       unsigned int i;
>> +
>> +       for (i = 0; i < folio_nr_pages(folio); i++)
>> +               clear_highpage(folio_page(folio, i));
>> +}
>> +
>> +static void swap_zeromap_folio_set(struct folio *folio)
>> +{
>> +       struct swap_info_struct *sis = swp_swap_info(folio->swap);
>> +       swp_entry_t entry;
>> +       unsigned int i;
>> +
>> +       for (i = 0; i < folio_nr_pages(folio); i++) {
>> +               entry = page_swap_entry(folio_page(folio, i));
>> +               set_bit(swp_offset(entry), sis->zeromap);
>> +       }
>> +}
>> +
>> +static void swap_zeromap_folio_clear(struct folio *folio)
>> +{
>> +       struct swap_info_struct *sis = swp_swap_info(folio->swap);
>> +       swp_entry_t entry;
>> +       unsigned int i;
>> +
>> +       for (i = 0; i < folio_nr_pages(folio); i++) {
>> +               entry = page_swap_entry(folio_page(folio, i));
>> +               clear_bit(swp_offset(entry), sis->zeromap);
>> +       }
>> +}
>> +
>> +static bool swap_zeromap_folio_test(struct folio *folio)
>> +{
>> +       struct swap_info_struct *sis = swp_swap_info(folio->swap);
>> +       swp_entry_t entry;
>> +       unsigned int i;
>> +
>> +       for (i = 0; i < folio_nr_pages(folio); i++) {
>> +               entry = page_swap_entry(folio_page(folio, i));
>> +               if (!test_bit(swp_offset(entry), sis->zeromap))
>> +                       return false;
>> +       }
>> +       return true;
>> +}
>> +
>>   /*
>>    * We may have stale swap cache pages in memory: notice
>>    * them here and get rid of the unnecessary final write.
>> @@ -195,6 +271,15 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
>>                  folio_unlock(folio);
>>                  return ret;
>>          }
>> +
>> +       if (is_folio_zero_filled(folio)) {
>> +               swap_zeromap_folio_set(folio);
>> +               folio_start_writeback(folio);
>> +               folio_unlock(folio);
>> +               folio_end_writeback(folio);
>> +               return 0;
>> +       }
>> +       swap_zeromap_folio_clear(folio);
>>          if (zswap_store(folio)) {
>>                  folio_start_writeback(folio);
>>                  folio_unlock(folio);
>> @@ -515,8 +600,11 @@ void swap_read_folio(struct folio *folio, bool synchronous,
>>                  psi_memstall_enter(&pflags);
>>          }
>>          delayacct_swapin_start();
>> -
>> -       if (zswap_load(folio)) {
>> +       if (swap_zeromap_folio_test(folio)) {
>> +               folio_zero_fill(folio);
>> +               folio_mark_uptodate(folio);
>> +               folio_unlock(folio);
>> +       } else if (zswap_load(folio)) {
>>                  folio_mark_uptodate(folio);
>>                  folio_unlock(folio);
>>          } else if (data_race(sis->flags & SWP_FS_OPS)) {
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index f1e559e216bd..90451174fe34 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -453,6 +453,8 @@ static unsigned int cluster_list_del_first(struct swap_cluster_list *list,
>>   static void swap_cluster_schedule_discard(struct swap_info_struct *si,
>>                  unsigned int idx)
>>   {
>> +       unsigned int i;
>> +
>>          /*
>>           * If scan_swap_map_slots() can't find a free cluster, it will check
>>           * si->swap_map directly. To make sure the discarding cluster isn't
>> @@ -461,6 +463,13 @@ static void swap_cluster_schedule_discard(struct swap_info_struct *si,
>>           */
>>          memset(si->swap_map + idx * SWAPFILE_CLUSTER,
>>                          SWAP_MAP_BAD, SWAPFILE_CLUSTER);
>> +       /*
>> +        * zeromap can see updates from concurrent swap_writepage() and swap_read_folio()
>> +        * call on other slots, hence use atomic clear_bit for zeromap instead of the
>> +        * non-atomic bitmap_clear.
>> +        */
>> +       for (i = 0; i < SWAPFILE_CLUSTER; i++)
>> +               clear_bit(idx * SWAPFILE_CLUSTER + i, si->zeromap);
>>
>>          cluster_list_add_tail(&si->discard_clusters, si->cluster_info, idx);
>>
>> @@ -482,7 +491,7 @@ static void __free_cluster(struct swap_info_struct *si, unsigned long idx)
>>   static void swap_do_scheduled_discard(struct swap_info_struct *si)
>>   {
>>          struct swap_cluster_info *info, *ci;
>> -       unsigned int idx;
>> +       unsigned int idx, i;
>>
>>          info = si->cluster_info;
>>
>> @@ -498,6 +507,8 @@ static void swap_do_scheduled_discard(struct swap_info_struct *si)
>>                  __free_cluster(si, idx);
>>                  memset(si->swap_map + idx * SWAPFILE_CLUSTER,
>>                                  0, SWAPFILE_CLUSTER);
>> +               for (i = 0; i < SWAPFILE_CLUSTER; i++)
>> +                       clear_bit(idx * SWAPFILE_CLUSTER + i, si->zeromap);
>>                  unlock_cluster(ci);
>>          }
>>   }
>> @@ -1336,6 +1347,7 @@ static void swap_entry_free(struct swap_info_struct *p, swp_entry_t entry)
>>          count = p->swap_map[offset];
>>          VM_BUG_ON(count != SWAP_HAS_CACHE);
>>          p->swap_map[offset] = 0;
>> +       clear_bit(offset, p->zeromap);
> Hmm so clear_bit() is done at the swap_entry_free() point. I wonder if
> we can have a problem, where:
>
> 1. The swap entry has its zeromap bit set, and is freed to the swap
> slot cache (free_swap_slot() in mm/swap_slots.c). For instance, it is
> reclaimed from the swap cache, and all the processes referring to it
> are terminated, which decrements the swap count to 0 (swap_free() ->
> __swap_entry_free() -> free_swap_slots())
>
> 2. The swap slot is then re-used in swap space allocation
> (add_to_swap()) - its zeromap bit is never cleared.
>
> 3. swap_writepage() writes that non-zero page to swap

In swap_writepage, with this patch you have:

     if (is_folio_zero_filled(folio)) {
         swap_zeromap_folio_set(folio);
         folio_unlock(folio);
         return 0;
     }
     swap_zeromap_folio_clear(folio);

i.e. if folio is not zero filled, swap_zeromap_folio_clear will be 
called and the bit is cleared, so I think it would take care of this 
scenario? swap_read_folio will see the bit cleared in step 4.

> 4. swap_read_folio() checks the bitmap, sees that the zeromap bit for
> the entry is set, so populates a zero page for it.
>
> zswap in the past has to carefully invalidate these leftover entries
> quite carefully. Chengming then move the invalidation point to
> free_swap_slot(), massively simplifying the logic.
>
> I wonder if we need to do the same here?