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

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Date: Fri, 12 Apr 2024 10:07:35 +0800
From: Chuanhua Han <chuanhuahan@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Barry Song <21cnbao@...il.com>, 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, surenb@...gle.com, v-songbaohua@...o.com, 
	willy@...radead.org, xiang@...nel.org, ying.huang@...el.com, 
	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()

Ryan Roberts <ryan.roberts@....com> 于2024年4月11日周四 22:30写道：
>
> On 09/04/2024 09:26, Barry Song wrote:
> > 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>
>
> Couple of nits; feel free to ignore.
>
> Reviewed-by: Ryan Roberts <ryan.roberts@....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, batched free swap entries
> > + * for this large folio, entry should be for the first subpage and
> > + * its offset is aligned with nr_pages
> > + */
> > +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;
> > +     }
> > +
> > +     remain_nr = nr_pages;
> > +     p = _swap_info_get(entry);
> > +     if (p) {
>
> nit: perhaps return early if (!p) ? Then you dedent the for() block.

Agreed!

>
> > +             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));
> > +
>
> nit: perhaps change to for (;;), and do the checks here to avoid clearing the
> bitmap on the last run:
>
>                         i += batch_nr;
>                         if (i < nr_pages)
>                                 break;
Great, thank you for your advice!
>
> > +                     bitmap_clear(usage, 0, SWAP_BATCH_NR);
> > +                     remain_nr -= batch_nr;
> > +             }
> > +     }
> > +}
> > +
> >  /*
> >   * Called after dropping swapcache to decrease refcnt to swap entries.
> >   */
>
>


-- 
Thanks,
Chuanhua