Message-ID: <CAGsJ_4yPQyQpWvB2KmuOh2vyUQA0A6hNHsypEC9Gptp9XtuDgg@mail.gmail.com>
Date: Mon, 15 Apr 2024 19:04:08 +1200
From: Barry Song <21cnbao@...il.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: 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, ryan.roberts@....com, surenb@...gle.com, 
	v-songbaohua@...o.com, willy@...radead.org, xiang@...nel.org, 
	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 Mon, Apr 15, 2024 at 6:19 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> Barry Song <21cnbao@...il.com> writes:
>
> > 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>
> > ---
> >  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,
>
> IMHO, it's not good to document the caller in the function definition.
> Because this will discourage function reusing.

ok. right now there is only one user that is why it is added. but i agree
we can actually remove this.

>
> > batched free swap entries
> > + * for this large folio, entry should be for the first subpage and
> > + * its offset is aligned with nr_pages
>
> Why do we need this?

This is a fundamental requirement for the existing kernel, folio's
swap offset is naturally aligned from the first moment add_to_swap
to add swapcache's xa. so this comment is describing the existing
fact. In the future, if we want to support swap-out folio to discontiguous
and not-aligned offsets, we can't pass entry as the parameter, we should
instead pass ptep or another different data struct which can connect
multiple discontiguous swap offsets.

I feel like we only need "for this large folio, entry should be for
the first subpage" and drop "and its offset is aligned with nr_pages",
the latter is not important to this context at all.

>
> > + */
> > +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;
> > +     }
>
> Is it possible to unify swap_free() and swap_free_nr() into one function
> with acceptable performance?  IIUC, the general rule in mTHP effort is
> to avoid duplicate functions between mTHP and normal small folio.
> Right?

I don't see why. but we have lots of places calling swap_free(), we may
have to change them all to call swap_free_nr(entry, 1); the other possible
way is making swap_free() a wrapper of swap_free_nr() always using
1 as the argument. In both cases, we are changing the semantics of
swap_free_nr() to partially freeing large folio cases and have to drop
"entry should be for the first subpage" then.

Right now, the semantics is
* swap_free_nr() for an entire large folio;
* swap_free() for one entry of either a large folio or a small folio

>
> > +
> > +     remain_nr = nr_pages;
> > +     p = _swap_info_get(entry);
> > +     if (p) {
> > +             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));
> > +
> > +                     bitmap_clear(usage, 0, SWAP_BATCH_NR);
> > +                     remain_nr -= batch_nr;
> > +             }
> > +     }
> > +}
> > +
> >  /*
> >   * Called after dropping swapcache to decrease refcnt to swap entries.
> >   */
>
> put_swap_folio() implements batching in another method.  Do you think
> that it's good to use the batching method in that function here?  It
> avoids to use bitmap operations and stack space.

Chuanhua has strictly limited the maximum stack usage to several
unsigned long, so this should be safe. on the other hand, i believe this
implementation is more efficient, as  put_swap_folio() might lock/
unlock much more often whenever __swap_entry_free_locked returns
0.

>
> --
> Best Regards,
> Huang, Ying

Thanks
Barry

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