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

[Barry Song]

On Mon, Apr 15, 2024 at 7:04 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Mon, Apr 15, 2024 at 6:19 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> >
> > Barry Song <21cnbao@xxxxxxxxx> writes:
> >
> > > 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 |  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.

upon further consideration, the comment is inaccurate since we do support
nr_pages == 1, and do_swap_page() has indeed been invoked with this value.
Therefore, we should completely remove the comment.

>
> >
> > > + */
> > > +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