Re: [PATCH 6/7] mm, swap: remove swap slot cache

[Kairui Song <ryncsn@gmail.com>]

On Thu, Feb 20, 2025 at 3:56 PM Baoquan He <bhe@redhat.com> wrote:
>
> Hi Kairui,
>
> On 02/15/25 at 01:57am, Kairui Song wrote:
> ......snip....
> > -int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_order)
> > +swp_entry_t folio_alloc_swap(struct folio *folio)
> >  {
> > -     int order = swap_entry_order(entry_order);
> > -     unsigned long size = 1 << order;
> > +     unsigned int order = folio_order(folio);
> > +     unsigned int size = 1 << order;
> >       struct swap_info_struct *si, *next;
> > -     int n_ret = 0;
> > +     swp_entry_t entry = {};
> > +     unsigned long offset;
> >       int node;
> >
> > +     if (order) {
> > +             /*
> > +              * Should not even be attempting large allocations when huge
> > +              * page swap is disabled. Warn and fail the allocation.
> > +              */
> > +             if (!IS_ENABLED(CONFIG_THP_SWAP) || size > SWAPFILE_CLUSTER) {
> > +                     VM_WARN_ON_ONCE(1);
> > +                     return entry;
> > +             }
> > +     }
> > +
> >       /* Fast path using percpu cluster */
> >       local_lock(&percpu_swap_cluster.lock);
> > -     n_ret = swap_alloc_fast(swp_entries,
> > -                             SWAP_HAS_CACHE,
> > -                             order, n_goal);
> > -     if (n_ret == n_goal)
> > -             goto out;
> > +     if (swap_alloc_fast(&entry, SWAP_HAS_CACHE, order))
> > +             goto out_alloced;
> >
> > -     n_goal = min_t(int, n_goal - n_ret, SWAP_BATCH);
> >       /* Rotate the device and switch to a new cluster */
> >       spin_lock(&swap_avail_lock);
> >  start_over:
> > @@ -1268,11 +1236,14 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_order)
> >               plist_requeue(&si->avail_lists[node], &swap_avail_heads[node]);
> >               spin_unlock(&swap_avail_lock);
> >               if (get_swap_device_info(si)) {
> > -                     n_ret += scan_swap_map_slots(si, SWAP_HAS_CACHE, n_goal,
> > -                                     swp_entries + n_ret, order);
> > +                     offset = cluster_alloc_swap_entry(si, order, SWAP_HAS_CACHE);
> >                       put_swap_device(si);
> > -                     if (n_ret || size > 1)
> > -                             goto out;
> > +                     if (offset) {
> > +                             entry = swp_entry(si->type, offset);
> > +                             goto out_alloced;
> > +                     }
> > +                     if (order)
> > +                             goto out_failed;
>
> This is not related to this patch, do you know why non order-0 case
> can't start over on different devices?

I think that might be an existing bug... I just didn change it as it's
kind of trivial, and also the comment "Swapfile is not block device so
unable to allocate large entries." which I didn't change either, is
also looking strange, but I prefer to fix them later as the background
seems a bit complex to explain.

>
> >               }
> >
> >               spin_lock(&swap_avail_lock);
> > @@ -1291,10 +1262,20 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_order)
> >                       goto start_over;
> >       }
> >       spin_unlock(&swap_avail_lock);
> > -out:
> > +out_failed:
> > +     local_unlock(&percpu_swap_cluster.lock);
> > +     return entry;
> > +
> > +out_alloced:
> >       local_unlock(&percpu_swap_cluster.lock);
> > -     atomic_long_sub(n_ret * size, &nr_swap_pages);
> > -     return n_ret;
> > +     if (mem_cgroup_try_charge_swap(folio, entry)) {
> > +             put_swap_folio(folio, entry);
> > +             entry.val = 0;
> > +     } else {
> > +             atomic_long_sub(size, &nr_swap_pages);
> > +     }
> > +
> > +     return entry;
> >  }
> >
> >  static struct swap_info_struct *_swap_info_get(swp_entry_t entry)
> ......snip....
> > @@ -2623,16 +2591,6 @@ static bool __has_usable_swap(void)
> >       return !plist_head_empty(&swap_active_head);
> >  }
>
> seems the __has_usable_swap() function need be moved into the ifdeffery
> scope where __folio_throttle_swaprate() is located to fix the lkp
> warning.

Yes, will fix the bot warning.


>
> >
> > -bool has_usable_swap(void)
> > -{
> > -     bool ret;
> > -
> > -     spin_lock(&swap_lock);
> > -     ret = __has_usable_swap();
> > -     spin_unlock(&swap_lock);
> > -     return ret;
> > -}
> > -
> >  /*
> >   * Called after clearing SWP_WRITEOK, ensures cluster_alloc_range
> >   * see the updated flags, so there will be no more allocations.
>
> Other than the test robot reported warning, this patch looks good to me.
> Thanks.
>
>