Re: [PATCH v2 11/15] mm, swap: use the swap table for the swap cache and switch API

[Kairui Song <ryncsn@gmail.com>]

On Mon, Sep 8, 2025 at 11:14 PM Kairui Song <ryncsn@gmail.com> wrote:
>
> On Mon, Sep 8, 2025 at 11:09 PM David Hildenbrand <david@redhat.com> wrote:
> > > +static inline struct swap_cluster_info *swap_cluster_lock(
> > > +     struct swap_info_struct *si, pgoff_t offset, bool irq)
> > > +{
> > > +     return NULL;
> > > +}
> > > +
> > > +static inline struct swap_cluster_info *swap_cluster_lock_by_folio(
> > > +             struct folio *folio)
> >
> > I would probably call that "swap_cluster_get_and_lock" or sth like that ...
> >
> > > +{
> > > +     return NULL;
> > > +}
> > > +
> > > +static inline struct swap_cluster_info *swap_cluster_lock_by_folio_irq(
> > > +             struct folio *folio)
> > > +{
> >
> > ... then this would become "swap_cluster_get_and_lock_irq"
> >
> >
> > Alterantively, separate the lookup from the locking of the cluster.
> >
> > swap_cluster_from_folio() / folio_get_swap_cluster()
> > swap_cluster_lock()
> > swap_cluster_lock_irq()
> >
> > Which might look cleaner in the end.
>
> That's a very good suggestion.

Hmm, one problem here is swap_cluster_lock's args are the `si` and
`offset`. But swap_cluster_from_folio returns a pointer to the
cluster. Retrieving the offset / si pointer after having the pointer
to the cluster seems redundant and troublesome.

I think maybe swap_cluster_get_and_lock is better. (Not sure if it
looks strange to take a folio as argument).

>
> >
> > [...]
> >
> > > -struct address_space *swapper_spaces[MAX_SWAPFILES] __read_mostly;
> > > -static unsigned int nr_swapper_spaces[MAX_SWAPFILES] __read_mostly;
> > > +struct address_space swap_space __read_mostly = {
> > > +     .a_ops = &swap_aops,
> > > +};
> > > +
> > >   static bool enable_vma_readahead __read_mostly = true;
> > >
> > >   #define SWAP_RA_ORDER_CEILING       5
> > > @@ -83,11 +86,21 @@ void show_swap_cache_info(void)
> > >    */
> > >   struct folio *swap_cache_get_folio(swp_entry_t entry)
> > >   {
> > > -     struct folio *folio = filemap_get_folio(swap_address_space(entry),
> > > -                                             swap_cache_index(entry));
> > > -     if (IS_ERR(folio))
> > > -             return NULL;
> > > -     return folio;
> > > +
> >
> > ^ superfluous empty line.
> >
> > [...]
> >
> > >
> > > @@ -420,6 +421,34 @@ static inline unsigned int cluster_offset(struct swap_info_struct *si,
> > >       return cluster_index(si, ci) * SWAPFILE_CLUSTER;
> > >   }
> > >
> > > +static int swap_table_alloc_table(struct swap_cluster_info *ci)
> >
> > swap_cluster_alloc_table ?
>
> Good idea.
>
> >
> > > +{
> > > +     WARN_ON(ci->table);
> > > +     ci->table = kzalloc(sizeof(unsigned long) * SWAPFILE_CLUSTER, GFP_KERNEL);
> > > +     if (!ci->table)
> > > +             return -ENOMEM;
> > > +     return 0;
> > > +}
> > > +
> > > +static void swap_cluster_free_table(struct swap_cluster_info *ci)
> > > +{
> > > +     unsigned int ci_off;
> > > +     unsigned long swp_tb;
> > > +
> > > +     if (!ci->table)
> > > +             return;
> > > +
> > > +     for (ci_off = 0; ci_off < SWAPFILE_CLUSTER; ci_off++) {
> > > +             swp_tb = __swap_table_get(ci, ci_off);
> > > +             if (!swp_tb_is_null(swp_tb))
> > > +                     pr_err_once("swap: unclean swap space on swapoff: 0x%lx",
> > > +                                 swp_tb);
> > > +     }
> > > +
> > > +     kfree(ci->table);
> > > +     ci->table = NULL;
> > > +}
> > > +
> > >   static void move_cluster(struct swap_info_struct *si,
> > >                        struct swap_cluster_info *ci, struct list_head *list,
> > >                        enum swap_cluster_flags new_flags)
> > > @@ -702,6 +731,26 @@ static bool cluster_scan_range(struct swap_info_struct *si,
> > >       return true;
> > >   }
> > >
> > > +/*
> > > + * Currently, the swap table is not used for count tracking, just
> > > + * do a sanity check here to ensure nothing leaked, so the swap
> > > + * table should be empty upon freeing.
> > > + */
> > > +static void cluster_table_check(struct swap_cluster_info *ci,
> > > +                             unsigned int start, unsigned int nr)
> >
> > "swap_cluster_assert_table_empty()"
> >
> > or sth like that that makes it clearer what you are checking for.
>
> Agree.
>
> >
> > --
> > Cheers
> >
>
> Thanks!