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

[Chuanhua Han]

Ryan Roberts <ryan.roberts@xxxxxxx> 于2024年4月12日周五 19:28写道：
>
> On 12/04/2024 03:07, Chuanhua Han wrote:
> > Ryan Roberts <ryan.roberts@xxxxxxx> 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@xxxxxxxx>
> >>>
> >>> 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@xxxxxxxx>
> >>> Co-developed-by: Barry Song <v-songbaohua@xxxxxxxx>
> >>> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> >>
> >> Couple of nits; feel free to ignore.
> >>
> >> Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> >>
> >>> ---
> >>>  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;
Should be：
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?
Yeah, that's OK, actually these two ways are similar, both are to
reduce the number of bitmap_clear calls.
>
> >>
> >>> +                     bitmap_clear(usage, 0, SWAP_BATCH_NR);
> >>> +                     remain_nr -= batch_nr;
> >>> +             }
> >>> +     }
> >>> +}
> >>> +
> >>>  /*
> >>>   * Called after dropping swapcache to decrease refcnt to swap entries.
> >>>   */
> >>
> >>
> >
> >
>


-- 
Thanks,
Chuanhua