Re: [PATCH v6 1/6] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags

[Chris Li <chrisl@kernel.org>]

On Thu, Apr 4, 2024 at 12:06 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 03/04/2024 23:12, Chris Li wrote:
> > Hi Ryan,
> >
> > Sorry for the late reply. I want to review this series but don't have
> > the chance to do it sooner.
>
> No problem. This series is now in mm-unstable, so if you want to request any
> changes in the other patches, I'd prefer it sooner rather than later, if possible.
>
> >
> > On Wed, Apr 3, 2024 at 4:40 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >>
> >> As preparation for supporting small-sized THP in the swap-out path,
> >> without first needing to split to order-0, Remove the CLUSTER_FLAG_HUGE,
> >> which, when present, always implies PMD-sized THP, which is the same as
> >> the cluster size.
> >>
> >> The only use of the flag was to determine whether a swap entry refers to
> >> a single page or a PMD-sized THP in swap_page_trans_huge_swapped().
> >> Instead of relying on the flag, we now pass in nr_pages, which
> >> originates from the folio's number of pages. This allows the logic to
> >> work for folios of any order.
> >>
> >> The one snag is that one of the swap_page_trans_huge_swapped() call
> >> sites does not have the folio. But it was only being called there to
> >> shortcut a call __try_to_reclaim_swap() in some cases.
> >> __try_to_reclaim_swap() gets the folio and (via some other functions)
> >> calls swap_page_trans_huge_swapped(). So I've removed the problematic
> >> call site and believe the new logic should be functionally equivalent.
> >>
> >> That said, removing the fast path means that we will take a reference
> >> and trylock a large folio much more often, which we would like to avoid.
> >> The next patch will solve this.
> >>
> >> Removing CLUSTER_FLAG_HUGE also means we can remove split_swap_cluster()
> >> which used to be called during folio splitting, since
> >> split_swap_cluster()'s only job was to remove the flag.
> >
> > Seems necessary to remove the assumption of large folio be PMD size.
> >
> > Acked-by: Chris Li <chrisl@kernel.org>
>
> Thanks!
>
> >
> >>
> >> Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
> >> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> >> ---
> >>  include/linux/swap.h | 10 ----------
> >>  mm/huge_memory.c     |  3 ---
> >>  mm/swapfile.c        | 47 ++++++++------------------------------------
> >>  3 files changed, 8 insertions(+), 52 deletions(-)
> >>
> >> diff --git a/include/linux/swap.h b/include/linux/swap.h
> >> index a211a0383425..f6f78198f000 100644
> >> --- a/include/linux/swap.h
> >> +++ b/include/linux/swap.h
> >> @@ -259,7 +259,6 @@ struct swap_cluster_info {
> >>  };
> >>  #define CLUSTER_FLAG_FREE 1 /* This cluster is free */
> >>  #define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */
> >> -#define CLUSTER_FLAG_HUGE 4 /* This cluster is backing a transparent huge page */
> >>
> >>  /*
> >>   * We assign a cluster to each CPU, so each CPU can allocate swap entry from
> >> @@ -590,15 +589,6 @@ static inline int add_swap_extent(struct swap_info_struct *sis,
> >>  }
> >>  #endif /* CONFIG_SWAP */
> >>
> >> -#ifdef CONFIG_THP_SWAP
> >> -extern int split_swap_cluster(swp_entry_t entry);
> >> -#else
> >> -static inline int split_swap_cluster(swp_entry_t entry)
> >> -{
> >> -       return 0;
> >> -}
> >> -#endif
> >> -
> >>  #ifdef CONFIG_MEMCG
> >>  static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
> >>  {
> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >> index ea6d1f09a0b9..3ca9282a0dc9 100644
> >> --- a/mm/huge_memory.c
> >> +++ b/mm/huge_memory.c
> >> @@ -2844,9 +2844,6 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> >>                 shmem_uncharge(folio->mapping->host, nr_dropped);
> >>         remap_page(folio, nr);
> >>
> >> -       if (folio_test_swapcache(folio))
> >> -               split_swap_cluster(folio->swap);
> >> -
> >>         /*
> >>          * set page to its compound_head when split to non order-0 pages, so
> >>          * we can skip unlocking it below, since PG_locked is transferred to
> >> diff --git a/mm/swapfile.c b/mm/swapfile.c
> >> index 5e6d2304a2a4..0d44ee2b4f9c 100644
> >> --- a/mm/swapfile.c
> >> +++ b/mm/swapfile.c
> >> @@ -343,18 +343,6 @@ static inline void cluster_set_null(struct swap_cluster_info *info)
> >>         info->data = 0;
> >>  }
> >>
> >> -static inline bool cluster_is_huge(struct swap_cluster_info *info)
> >> -{
> >> -       if (IS_ENABLED(CONFIG_THP_SWAP))
> >> -               return info->flags & CLUSTER_FLAG_HUGE;
> >> -       return false;
> >> -}
> >> -
> >> -static inline void cluster_clear_huge(struct swap_cluster_info *info)
> >> -{
> >> -       info->flags &= ~CLUSTER_FLAG_HUGE;
> >> -}
> >> -
> >>  static inline struct swap_cluster_info *lock_cluster(struct swap_info_struct *si,
> >>                                                      unsigned long offset)
> >>  {
> >> @@ -1027,7 +1015,7 @@ static int swap_alloc_cluster(struct swap_info_struct *si, swp_entry_t *slot)
> >>         offset = idx * SWAPFILE_CLUSTER;
> >>         ci = lock_cluster(si, offset);
> >>         alloc_cluster(si, idx);
> >> -       cluster_set_count_flag(ci, SWAPFILE_CLUSTER, CLUSTER_FLAG_HUGE);
> >> +       cluster_set_count(ci, SWAPFILE_CLUSTER);
> >>
> >>         memset(si->swap_map + offset, SWAP_HAS_CACHE, SWAPFILE_CLUSTER);
> >>         unlock_cluster(ci);
> >> @@ -1365,7 +1353,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
> >>
> >>         ci = lock_cluster_or_swap_info(si, offset);
> >>         if (size == SWAPFILE_CLUSTER) {
> >> -               VM_BUG_ON(!cluster_is_huge(ci));
> >>                 map = si->swap_map + offset;
> >>                 for (i = 0; i < SWAPFILE_CLUSTER; i++) {
> >>                         val = map[i];
> >> @@ -1373,7 +1360,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
> >>                         if (val == SWAP_HAS_CACHE)
> >>                                 free_entries++;
> >>                 }
> >> -               cluster_clear_huge(ci);
> >>                 if (free_entries == SWAPFILE_CLUSTER) {
> >>                         unlock_cluster_or_swap_info(si, ci);
> >>                         spin_lock(&si->lock);
> >> @@ -1395,23 +1381,6 @@ void put_swap_folio(struct folio *folio, swp_entry_t entry)
> >>         unlock_cluster_or_swap_info(si, ci);
> >>  }
> >>
> >> -#ifdef CONFIG_THP_SWAP
> >> -int split_swap_cluster(swp_entry_t entry)
> >> -{
> >> -       struct swap_info_struct *si;
> >> -       struct swap_cluster_info *ci;
> >> -       unsigned long offset = swp_offset(entry);
> >> -
> >> -       si = _swap_info_get(entry);
> >> -       if (!si)
> >> -               return -EBUSY;
> >> -       ci = lock_cluster(si, offset);
> >> -       cluster_clear_huge(ci);
> >> -       unlock_cluster(ci);
> >> -       return 0;
> >> -}
> >> -#endif
> >> -
> >>  static int swp_entry_cmp(const void *ent1, const void *ent2)
> >>  {
> >>         const swp_entry_t *e1 = ent1, *e2 = ent2;
> >> @@ -1519,22 +1488,23 @@ int swp_swapcount(swp_entry_t entry)
> >>  }
> >>
> >>  static bool swap_page_trans_huge_swapped(struct swap_info_struct *si,
> >> -                                        swp_entry_t entry)
> >> +                                        swp_entry_t entry,
> >> +                                        unsigned int nr_pages)
> >>  {
> >>         struct swap_cluster_info *ci;
> >>         unsigned char *map = si->swap_map;
> >>         unsigned long roffset = swp_offset(entry);
> >> -       unsigned long offset = round_down(roffset, SWAPFILE_CLUSTER);
> >> +       unsigned long offset = round_down(roffset, nr_pages);
> >
> > It is obvious this code only works for powers two nr_pages. The
> > SWAPFILE_CLSTER is a power of two. If we switch to an API for
> > nr_pages, we might want to warn/ban passing in the non-power of two
> > nr_pages.
>
> Indeed. I could change the prototype to pass order instead of nr_pages, then
> generate nr_pages (= 1 << order) inside the function. But given the function is
> static and only called from a single callsite, I don't see it as hugely
> important. I'd prefer to leave as is at this stage, unless you have strong
> objection.

That is fine with me.

>
> >
> >>         int i;
> >>         bool ret = false;
> >>
> >>         ci = lock_cluster_or_swap_info(si, offset);
> >> -       if (!ci || !cluster_is_huge(ci)) {
> >> +       if (!ci || nr_pages == 1) {
> >>                 if (swap_count(map[roffset]))
> >>                         ret = true;
> >>                 goto unlock_out;
> >>         }
> >> -       for (i = 0; i < SWAPFILE_CLUSTER; i++) {
> >> +       for (i = 0; i < nr_pages; i++) {
> >
> > Here we assume the swap entry offset is contiguous. That is beyond
> > your patch's scope. If in the future we want to have non-contiguous
> > swap entries to swap out large pages, we will need to find out and
> > change all the places that have the assumption of contiguous swap
> > entries.
>
> Yes there are tonnes of places that make this assumption :)

Yes, that is why I want to have some wrapper API to mark the existing
place that makes the assumption. Hopefully we can just change those
wrapper functions to implement the non-contiguous version of swap.

Thank you for the patch.

Chris