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

[Huang, Ying]

Barry Song <21cnbao@xxxxxxxxx> writes:

> 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.

No.  In put_swap_folio(), free_entries is either SWAPFILE_CLUSTER (that
is, nr_pages) or 0.  These are the most common cases.

> 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.

Please consider this too.

--
Best Regards,
Huang, Ying