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

From: Barry Song
Date: Mon Apr 15 2024 - 05:01:42 EST

On Mon, Apr 15, 2024 at 8:53 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> 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.
>

i am actually talking about the below code path,

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);
}

but i guess you are talking about the below code path:

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

ci = lock_cluster_or_swap_info(si, offset);
if (size == SWAPFILE_CLUSTER) {
map = si->swap_map + offset;
for (i = 0; i < SWAPFILE_CLUSTER; i++) {
val = map[i];
VM_BUG_ON(!(val & SWAP_HAS_CACHE));
if (val == SWAP_HAS_CACHE)
free_entries++;
}
if (free_entries == SWAPFILE_CLUSTER) {
unlock_cluster_or_swap_info(si, ci);
spin_lock(&si->lock);
mem_cgroup_uncharge_swap(entry, SWAPFILE_CLUSTER);
swap_free_cluster(si, idx);
spin_unlock(&si->lock);
return;
}
}
}

we are mTHP, so we can't assume our size is SWAPFILE_CLUSTER?
or you want to check free_entries == "1 << swap_entry_order(folio_order(folio))"
instead of SWAPFILE_CLUSTER for the "for (i = 0; i < size; i++, entry.val++)"
path?


> >>
> >> And, we should add batching in __swap_entry_free(). That will help
> >> free_swap_and_cache_nr() too.

Chris Li and I actually discussed it before, while I completely
agree this can be batched. but i'd like to defer this as an incremental
patchset later to keep this swapcache-refault small.

>
> Please consider this too.
>
> --
> Best Regards,
> Huang, Ying