Re: [PATCH RFC 2/6] mm: swap: introduce swap_nr_free() for batched swap_free()

[Barry Song]

Hi Chris,

Thanks!

On Sat, Jan 27, 2024 at 12:17 PM Chris Li <chrisl@xxxxxxxxxx> wrote:
>
> On Thu, Jan 18, 2024 at 3:11 AM Barry Song <21cnbao@xxxxxxxxx> 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>
> > ---
> >  include/linux/swap.h |  6 ++++++
> >  mm/swapfile.c        | 29 +++++++++++++++++++++++++++++
> >  2 files changed, 35 insertions(+)
> >
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 4db00ddad261..31a4ee2dcd1c 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -478,6 +478,7 @@ 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_nr_free(swp_entry_t entry, int nr_pages);
> >  extern void swapcache_free_entries(swp_entry_t *entries, int n);
> >  extern int free_swap_and_cache(swp_entry_t);
> >  int swap_type_of(dev_t device, sector_t offset);
> > @@ -553,6 +554,11 @@ static inline void swap_free(swp_entry_t swp)
> >  {
> >  }
> >
> > +void swap_nr_free(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 556ff7347d5f..6321bda96b77 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -1335,6 +1335,35 @@ void swap_free(swp_entry_t entry)
> >                 __swap_entry_free(p, entry);
> >  }
> >
> > +void swap_nr_free(swp_entry_t entry, int nr_pages)
> > +{
> > +       int i;
> > +       struct swap_cluster_info *ci;
> > +       struct swap_info_struct *p;
> > +       unsigned type = swp_type(entry);
> > +       unsigned long offset = swp_offset(entry);
> > +       DECLARE_BITMAP(usage, SWAPFILE_CLUSTER) = { 0 };
> > +
> > +       VM_BUG_ON(offset % SWAPFILE_CLUSTER + nr_pages > SWAPFILE_CLUSTER);
>
> BUG_ON here seems a bit too developer originated. Maybe warn once and
> roll back to free one by one?

The function is used only for the case we are quite sure we are freeing
some contiguous swap entries within a cluster. if it is not the case,
we will need an array of entries[]. will people be more comfortable to
have a WARN_ON instead? but the problem is if that really happens,
it is a bug, WARN isn't enough.

>
> How big is your typical SWAPFILE_CUSTER and nr_pages typically in arm?

My case is SWAPFILE_CLUSTER  = HPAGE_PMD_NR = 2MB/4KB = 512.

>
> I ask this question because nr_ppages > 64, that is a totally
> different game, we can completely  bypass the swap cache slots.
>

I agree we have a chance to bypass slot cache if nr_pages is bigger than
SWAP_SLOTS_CACHE_SIZE. on the other hand, even when nr_pages <
64, we still have a good chance to optimize free_swap_slot() by batching
as there are many spin_lock and sort() for each single entry.


> > +
> > +       if (nr_pages == 1) {
> > +               swap_free(entry);
> > +               return;
> > +       }
> > +
> > +       p = _swap_info_get(entry);
> > +
> > +       ci = lock_cluster(p, offset);
> > +       for (i = 0; i < nr_pages; i++) {
> > +               if (__swap_entry_free_locked(p, offset + i, 1))
> > +                       __bitmap_set(usage, i, 1);
> > +       }
> > +       unlock_cluster(ci);
> > +
> > +       for_each_clear_bit(i, usage, nr_pages)
> > +               free_swap_slot(swp_entry(type, offset + i));
>
> Notice that free_swap_slot() internal has per CPU cache batching as
> well. Every free_swap_slot will get some per_cpu swap slot cache and
> cache->lock. There is double batching here.
> If the typical batch size here is bigger than 64 entries, we can go
> directly to batching swap_entry_free and avoid the free_swap_slot()
> batching altogether. Unlike free_swap_slot_entries(), here swap slots
> are all from one swap device, there is no need to sort and group the
> swap slot by swap devices.

I agree.  you are completely right!
However, to make the patchset smaller at the beginning, I prefer
these optimizations to be deferred as a separate patchset after this one.

>
> Chris
>
>
> Chris
>
> > +}
> > +
> >  /*
> >   * Called after dropping swapcache to decrease refcnt to swap entries.
> >   */
> > --
> > 2.34.1

Thanks
Barry