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Message-ID: <CAGsJ_4wLvRNx0dHvoS3Vc+d19BmfioceJdmPm5Ge+Kj55q3Lsw@mail.gmail.com>
Date: Mon, 15 Apr 2024 20:06:03 +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 7:04 PM Barry Song <21cnbao@...il.com> wrote:
>
> 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.
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
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