Message-ID: <CAGsJ_4wTsJe2yK3k+Tif6obgL9LUUuqXEBxhqMSZSFZpbvrzmA@mail.gmail.com>
Date: Wed, 17 Apr 2024 13:35:17 +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 Wed, Apr 17, 2024 at 12:34 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> Barry Song <21cnbao@...il.com> writes:
>
> > On Tue, Apr 16, 2024 at 3:13 PM Huang, Ying <ying.huang@...el.com> wrote:
> >>
> >> Barry Song <21cnbao@...il.com> writes:
> >>
> >> > On Tue, Apr 16, 2024 at 1:42 PM Huang, Ying <ying.huang@...el.com> wrote:
> >> >>
> >> >> Barry Song <21cnbao@...il.com> writes:
> >> >>
> >> >> > On Mon, Apr 15, 2024 at 8:53 PM Huang, Ying <ying.huang@...el.com> wrote:
> >> >> >>
> >> >> >> Barry Song <21cnbao@...il.com> writes:
> >> >> >>
> >> >> >> > On Mon, Apr 15, 2024 at 8:21 PM Huang, Ying <ying.huang@...el.com> wrote:
> >> >> >> >>
> >> >> >> >> Barry Song <21cnbao@...il.com> writes:
> >> >> >> >>
> >> >> >> >> > 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.
> >> >> >> >>
> >> >> >> >> 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;
> >> >> >                 }
> >> >> >         }
> >> >> > }
> >> >>
> >> >> I am talking about both code paths.  In 2 most common cases,
> >> >> __swap_entry_free_locked() will return 0 or !0 for all entries in range.
> >> >
> >> > I grasp your point, but if conditions involving 0 or non-0 values fail, we'll
> >> > end up repeatedly unlocking and locking. Picture a scenario with a large
> >> > folio shared by multiple processes. One process might unmap a portion
> >> > while another still holds an entire mapping to it. This could lead to situations
> >> > where free_entries doesn't equal 0 and free_entries doesn't equal
> >> > nr_pages, resulting in multiple unlock and lock operations.
> >>
> >> This is impossible in current caller, because the folio is in the swap
> >> cache.  But if we move the change to __swap_entry_free_nr(), we may run
> >> into this situation.
> >
> > I don't understand why it is impossible, after try_to_unmap_one() has done
> > on one process, mprotect and munmap called on a part of the large folio
> > pte entries which now have been swap entries, we are removing the PTE
> > for this part. Another process can entirely hit the swapcache and have
> > all swap entries mapped there, and we call swap_free_nr(entry, nr_pages) in
> > do_swap_page. Within those swap entries, some have swapcount=1 and others
> > have swapcount > 1. Am I missing something?
>
> For swap entries with swapcount=1, its sis->swap_map[] will be
>
> 1 | SWAP_HAS_CACHE
>
> so, __swap_entry_free_locked() will return SWAP_HAS_CACHE instead of 0.
>
> The swap entries will be free in
>
> folio_free_swap
>   delete_from_swap_cache
>     put_swap_folio
>

Yes. I realized this after replying to you yesterday.

> >> > Chuanhua has invested significant effort in following Ryan's suggestion
> >> > for the current approach, which generally handles all cases, especially
> >> > partial unmapping. Additionally, the widespread use of swap_free_nr()
> >> > as you suggested across various scenarios is noteworthy.
> >> >
> >> > Unless there's evidence indicating performance issues or bugs, I believe
> >> > the current approach remains preferable.
> >>
> >> TBH, I don't like the large stack space usage (64 bytes).  How about use
> >> a "unsigned long" as bitmap?  Then, we use much less stack space, use
> >> bitmap == 0 and bitmap == (unsigned long)(-1) to check the most common
> >> use cases.  And, we have enough batching.
> >
> > that is quite a straightforward modification like,
> >
> > - #define SWAP_BATCH_NR (SWAPFILE_CLUSTER > 512 ? 512 : SWAPFILE_CLUSTER)
> > + #define SWAP_BATCH_NR (SWAPFILE_CLUSTER > 64 ? 64 : SWAPFILE_CLUSTER)
> >
> > there is no necessity to remove the bitmap API and move to raw
> > unsigned long operations.
> > as bitmap is exactly some unsigned long. on 64bit CPU, we are now one
> > unsigned long,
> > on 32bit CPU, it is now two unsigned long.
>
> Yes.  We can still use most bitmap APIs if we use "unsigned long" as
> bitmap.  The advantage of "unsigned long" is to guarantee that
> bitmap_empty() and bitmap_full() is trivial.  We can use that for
> optimization.  For example, we can skip unlock/lock if bitmap_empty().

anyway we have avoided lock_cluster_or_swap_info and unlock_cluster_or_swap_info
for each individual swap entry.

if bitma_empty(), we won't call free_swap_slot() so no chance to
further take any lock,
right?

the optimization of bitmap_full() seems to be more useful only after we have
void free_swap_slot(swp_entry_t entry, int nr)

in which we can avoid many spin_lock_irq(&cache->free_lock);

On the other hand, it seems we can directly call
swapcache_free_entries() to skip cache if
nr_pages >= SWAP_BATCH(64) this might be an optimization as we are now
having a bitmap exactly equals 64.

>
> >>
> >> >>
> >> >> > 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?
> >> >>
> >> >> Just replace SWAPFILE_CLUSTER with "nr_pages" in your code.
> >> >>
> >> >> >
> >> >> >> >>
> >> >> >> >> 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.
> >> >>
> >> >> OK.
> >> >>
> >> >> >>
> >> >> >> Please consider this too.
>
> --
> Best Regards,
> Huang, Ying

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