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

[Ryan Roberts]

On 04/04/2024 14:43, Chris Li wrote:
> On Thu, Apr 4, 2024 at 12:06 AM Ryan Roberts <ryan.roberts@xxxxxxx> 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@xxxxxxx> 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@xxxxxxxxxx>
>>
>> Thanks!
>>
>>>
>>>>
>>>> Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>>>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>>>> ---
>>>>  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.

Given David spotted a bug in a later patch, I'm re-spinning the series, so will
make this change to pass the order.

> 
>>
>>>
>>>>         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