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

[Barry Song]

On Mon, Apr 15, 2024 at 8:21 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Barry Song <21cnbao@xxxxxxxxx> writes:
>
> > 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.
>
> IIUC, all these are requirements of the only caller now, not the
> function itself.  If only part of the all swap entries of a mTHP are
> called with swap_free_nr(), can swap_free_nr() still do its work?  If
> so, why not make swap_free_nr() as general as possible?

right , i believe we can make swap_free_nr() as general as possible.

>
> >>
> >> > + */
> >> > +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.
>
> Because duplicated implementation are hard to maintain in the long term.

sorry, i actually meant "I don't see why not",  for some reason, the "not"
was missed. Obviously I meant "why not", there was a "but" after it :-)

>
> > 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
>
> As above, I don't think the these semantics are important for
> swap_free_nr() implementation.

right. I agree. If we are ready to change all those callers, nothing
can stop us from removing swap_free().

>
> >>
> >> > +
> >> > +     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,
>
> 512 / 8 = 64 bytes.
>
> So, not trivial.
>
> > 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.
>
> There are 2 most common use cases,
>
> - all swap entries have usage count == 0
> - all swap entries have usage count != 0
>
> In both cases, we only need to lock/unlock once.  In fact, I didn't
> find possible use cases other than above.

i guess the point is free_swap_slot() shouldn't be called within
lock_cluster_or_swap_info? so when we are freeing nr_pages slots,
we'll have to unlock and lock nr_pages times?  and this is the most
common scenario.

void put_swap_folio(struct folio *folio, swp_entry_t entry)
{
        ...

        ci = lock_cluster_or_swap_info(si, offset);
        ...
        for (i = 0; i < size; i++, entry.val++) {
                if (!__swap_entry_free_locked(si, offset + i, SWAP_HAS_CACHE)) {
                        unlock_cluster_or_swap_info(si, ci);
                        free_swap_slot(entry);
                        if (i == size - 1)
                                return;
                        lock_cluster_or_swap_info(si, offset);
                }
        }
        unlock_cluster_or_swap_info(si, ci);
}

>
> And, we should add batching in __swap_entry_free().  That will help
> free_swap_and_cache_nr() too.
>
> --
> Best Regards,
> Huang, Ying