Re: [RFC PATCH v3 2/5] mm: swap: introduce swap_nr_free() for batched swap_free()

[Chuanhua Han]

Ryan Roberts <ryan.roberts@xxxxxxx> 于2024年3月12日周二 02:51写道：
>
> On 04/03/2024 08:13, Barry Song wrote:
> > 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 |  6 ++++++
> >  mm/swapfile.c        | 35 +++++++++++++++++++++++++++++++++++
> >  2 files changed, 41 insertions(+)
> >
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 2955f7a78d8d..d6ab27929458 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -481,6 +481,7 @@ 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_nr_free(swp_entry_t entry, int nr_pages);
>
> nit: In my swap-out v4 series, I've created a batched version of
> free_swap_and_cache() and called it free_swap_and_cache_nr(). Perhaps it is
> preferable to align the naming schemes - i.e. call this swap_free_nr(). Your
> scheme doesn't really work when applied to free_swap_and_cache().
Thanks for your suggestions, and for the next version, we'll see which
package is more appropriate!
>
> >  extern void swapcache_free_entries(swp_entry_t *entries, int n);
> >  extern int free_swap_and_cache(swp_entry_t);
> >  int swap_type_of(dev_t device, sector_t offset);
> > @@ -561,6 +562,11 @@ static inline void swap_free(swp_entry_t swp)
> >  {
> >  }
> >
> > +void swap_nr_free(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 3f594be83b58..244106998a69 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -1341,6 +1341,41 @@ void swap_free(swp_entry_t entry)
> >               __swap_entry_free(p, entry);
> >  }
> >
> > +/*
> > + * 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_nr_free(swp_entry_t entry, int nr_pages)
> > +{
> > +     int i;
> > +     struct swap_cluster_info *ci;
> > +     struct swap_info_struct *p;
> > +     unsigned type = swp_type(entry);
>
> nit: checkpatch.py will complain about bare "unsigned", preferring "unsigned
> int" or at least it did for me when I did something similar in my swap-out patch
> set.
Gee, thanks for pointing that out!
>
> > +     unsigned long offset = swp_offset(entry);
> > +     DECLARE_BITMAP(usage, SWAPFILE_CLUSTER) = { 0 };
>
> I don't love this, as it could blow the stack if SWAPFILE_CLUSTER ever
> increases. But the only other way I can think of is to explicitly loop over
> fixed size chunks, and that's not much better.
Is it possible to save kernel stack better by using bit_map here?  If
SWAPFILE_CLUSTER=512, we consume only (512/64)*8= 64 bytes.
>
> > +
> > +     /* 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;
> > +     }
> > +
> > +     p = _swap_info_get(entry);
>
> You need to handle this returning NULL, like swap_free() does.
Yes, you're right! We did forget to judge NULL here.
>
> > +
> > +     ci = lock_cluster(p, offset);
>
> The existing swap_free() calls lock_cluster_or_swap_info(). So if swap is backed
> by rotating media, and clusters are not in use, it will lock the whole swap
> info. But your new version only calls lock_cluster() which won't lock anything
> if clusters are not in use. So I think this is a locking bug.
Again, you're right, it's bug!
>
> > +     for (i = 0; i < nr_pages; i++) {
> > +             if (__swap_entry_free_locked(p, offset + i, 1))
> > +                     __bitmap_set(usage, i, 1);
> > +     }
> > +     unlock_cluster(ci);
> > +
> > +     for_each_clear_bit(i, usage, nr_pages)
> > +             free_swap_slot(swp_entry(type, offset + i));
> > +}
> > +
> >  /*
> >   * Called after dropping swapcache to decrease refcnt to swap entries.
> >   */
>
> Thanks,
> Ryan
>
>


-- 
Thanks,
Chuanhua