Re: [PATCH v7 2/7] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()

[Ryan Roberts]

On 09/04/2024 10:41, Barry Song wrote:
> On Tue, Apr 9, 2024 at 9:24 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>> On 09.04.24 11:22, Barry Song wrote:
>>> On Tue, Apr 9, 2024 at 8:51 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>>>>
>>>> On Tue, Apr 9, 2024 at 6:40 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>>>>
>>>>> Now that we no longer have a convenient flag in the cluster to determine
>>>>> if a folio is large, free_swap_and_cache() will take a reference and
>>>>> lock a large folio much more often, which could lead to contention and
>>>>> (e.g.) failure to split large folios, etc.
>>>>>
>>>>> Let's solve that problem by batch freeing swap and cache with a new
>>>>> function, free_swap_and_cache_nr(), to free a contiguous range of swap
>>>>> entries together. This allows us to first drop a reference to each swap
>>>>> slot before we try to release the cache folio. This means we only try to
>>>>> release the folio once, only taking the reference and lock once - much
>>>>> better than the previous 512 times for the 2M THP case.
>>>>>
>>>>> Contiguous swap entries are gathered in zap_pte_range() and
>>>>> madvise_free_pte_range() in a similar way to how present ptes are
>>>>> already gathered in zap_pte_range().
>>>>>
>>>>> While we are at it, let's simplify by converting the return type of both
>>>>> functions to void. The return value was used only by zap_pte_range() to
>>>>> print a bad pte, and was ignored by everyone else, so the extra
>>>>> reporting wasn't exactly guaranteed. We will still get the warning with
>>>>> most of the information from get_swap_device(). With the batch version,
>>>>> we wouldn't know which pte was bad anyway so could print the wrong one.
>>>>>
>>>>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>>>>> ---
>>>>>   include/linux/pgtable.h | 29 ++++++++++++
>>>>>   include/linux/swap.h    | 12 +++--
>>>>>   mm/internal.h           | 63 ++++++++++++++++++++++++++
>>>>>   mm/madvise.c            | 12 +++--
>>>>>   mm/memory.c             | 13 +++---
>>>>>   mm/swapfile.c           | 97 +++++++++++++++++++++++++++++++++--------
>>>>>   6 files changed, 195 insertions(+), 31 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>>>> index a3fc8150b047..75096025fe52 100644
>>>>> --- a/include/linux/pgtable.h
>>>>> +++ b/include/linux/pgtable.h
>>>>> @@ -708,6 +708,35 @@ static inline void pte_clear_not_present_full(struct mm_struct *mm,
>>>>>   }
>>>>>   #endif
>>>>>
>>>>> +#ifndef clear_not_present_full_ptes
>>>>> +/**
>>>>> + * clear_not_present_full_ptes - Clear multiple not present PTEs which are
>>>>> + *                              consecutive in the pgtable.
>>>>> + * @mm: Address space the ptes represent.
>>>>> + * @addr: Address of the first pte.
>>>>> + * @ptep: Page table pointer for the first entry.
>>>>> + * @nr: Number of entries to clear.
>>>>> + * @full: Whether we are clearing a full mm.
>>>>> + *
>>>>> + * May be overridden by the architecture; otherwise, implemented as a simple
>>>>> + * loop over pte_clear_not_present_full().
>>>>> + *
>>>>> + * Context: The caller holds the page table lock.  The PTEs are all not present.
>>>>> + * The PTEs are all in the same PMD.
>>>>> + */
>>>>> +static inline void clear_not_present_full_ptes(struct mm_struct *mm,
>>>>> +               unsigned long addr, pte_t *ptep, unsigned int nr, int full)
>>>>> +{
>>>>> +       for (;;) {
>>>>> +               pte_clear_not_present_full(mm, addr, ptep, full);
>>>>> +               if (--nr == 0)
>>>>> +                       break;
>>>>> +               ptep++;
>>>>> +               addr += PAGE_SIZE;
>>>>> +       }
>>>>> +}
>>>>> +#endif
>>>>> +
>>>>>   #ifndef __HAVE_ARCH_PTEP_CLEAR_FLUSH
>>>>>   extern pte_t ptep_clear_flush(struct vm_area_struct *vma,
>>>>>                                unsigned long address,
>>>>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>>>>> index f6f78198f000..5737236dc3ce 100644
>>>>> --- a/include/linux/swap.h
>>>>> +++ b/include/linux/swap.h
>>>>> @@ -471,7 +471,7 @@ extern int swap_duplicate(swp_entry_t);
>>>>>   extern int swapcache_prepare(swp_entry_t);
>>>>>   extern void swap_free(swp_entry_t);
>>>>>   extern void swapcache_free_entries(swp_entry_t *entries, int n);
>>>>> -extern int free_swap_and_cache(swp_entry_t);
>>>>> +extern void free_swap_and_cache_nr(swp_entry_t entry, int nr);
>>>>>   int swap_type_of(dev_t device, sector_t offset);
>>>>>   int find_first_swap(dev_t *device);
>>>>>   extern unsigned int count_swap_pages(int, int);
>>>>> @@ -520,8 +520,9 @@ static inline void put_swap_device(struct swap_info_struct *si)
>>>>>   #define free_pages_and_swap_cache(pages, nr) \
>>>>>          release_pages((pages), (nr));
>>>>>
>>>>> -/* used to sanity check ptes in zap_pte_range when CONFIG_SWAP=0 */
>>>>> -#define free_swap_and_cache(e) is_pfn_swap_entry(e)
>>>>> +static inline void free_swap_and_cache_nr(swp_entry_t entry, int nr)
>>>>> +{
>>>>> +}
>>>>>
>>>>>   static inline void free_swap_cache(struct folio *folio)
>>>>>   {
>>>>> @@ -589,6 +590,11 @@ static inline int add_swap_extent(struct swap_info_struct *sis,
>>>>>   }
>>>>>   #endif /* CONFIG_SWAP */
>>>>>
>>>>> +static inline void free_swap_and_cache(swp_entry_t entry)
>>>>> +{
>>>>> +       free_swap_and_cache_nr(entry, 1);
>>>>> +}
>>>>> +
>>>>>   #ifdef CONFIG_MEMCG
>>>>>   static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
>>>>>   {
>>>>> diff --git a/mm/internal.h b/mm/internal.h
>>>>> index 3bdc8693b54f..de68705624b0 100644
>>>>> --- a/mm/internal.h
>>>>> +++ b/mm/internal.h
>>>>> @@ -11,6 +11,8 @@
>>>>>   #include <linux/mm.h>
>>>>>   #include <linux/pagemap.h>
>>>>>   #include <linux/rmap.h>
>>>>> +#include <linux/swap.h>
>>>>> +#include <linux/swapops.h>
>>>>>   #include <linux/tracepoint-defs.h>
>>>>>
>>>>>   struct folio_batch;
>>>>> @@ -189,6 +191,67 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>>>>>
>>>>>          return min(ptep - start_ptep, max_nr);
>>>>>   }
>>>>> +
>>>>> +/**
>>>>> + * pte_next_swp_offset - Increment the swap entry offset field of a swap pte.
>>>>> + * @pte: The initial pte state; is_swap_pte(pte) must be true.
>>>>> + *
>>>>> + * Increments the swap offset, while maintaining all other fields, including
>>>>> + * swap type, and any swp pte bits. The resulting pte is returned.
>>>>> + */
>>>>> +static inline pte_t pte_next_swp_offset(pte_t pte)
>>>>> +{
>>>>> +       swp_entry_t entry = pte_to_swp_entry(pte);
>>>>> +       pte_t new = __swp_entry_to_pte(__swp_entry(swp_type(entry),
>>>>> +                                                  swp_offset(entry) + 1));
>>>>> +
>>>>> +       if (pte_swp_soft_dirty(pte))
>>>>> +               new = pte_swp_mksoft_dirty(new);
>>>>> +       if (pte_swp_exclusive(pte))
>>>>> +               new = pte_swp_mkexclusive(new);
>>>>> +       if (pte_swp_uffd_wp(pte))
>>>>> +               new = pte_swp_mkuffd_wp(new);
>>>>
>>>> I don't quite understand this. If this page table entry is exclusive,
>>>> will its subsequent page table entry also be exclusive without
>>>> question?
>>>> in try_to_unmap_one, exclusive is per-subpage but not per-folio:
>>>>
>>>>                  anon_exclusive = folio_test_anon(folio) &&
>>>>                                   PageAnonExclusive(subpage);
>>>>
>>>> same questions also for diry, wp etc.
>>>
>>> Sorry for the noise. you are right. based on your new version, I think I should
>>> entirely drop:
>>>
>>> [PATCH v2 3/5] mm: swap_pte_batch: add an output argument to reture if
>>> all swap entries are exclusive
>>
>> Yes. If we ever want to ignore some bits, we should likely add flags to
>> change the behavior, like for folio_pte_batch().
>>
>> For swapin, you really want the exclusive bits to match, though.
> 
> I am not quite sure I definitely need exclusive bits to match. i can either
> drop my 3/5 or ignore the exclusive bit as below (if anyone is not shared,
> swpin won't reuse the large folio, but it can still entirely map it read-only):
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index cae39c372bfc..5726e729c9ee 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -253,10 +253,22 @@ static inline int swap_pte_batch(pte_t
> *start_ptep, int max_nr, pte_t pte,
>                 *any_shared |= !pte_swp_exclusive(pte);
> 
>         while (ptep < end_ptep) {
> +               pte_t ignore_exclusive_pte;
> +               pte_t ignore_exclusive_expected_pte;
>                 pte = ptep_get(ptep);
> 
> -               if (!pte_same(pte, expected_pte))
> -                       break;
> +               if (any_shared) {
> +                       ignore_exclusive_pte = pte;
> +                       ignore_exclusive_expected_pte = expected_pte;
> +                       ignore_exclusive_pte =
> pte_swp_clear_exclusive(ignore_exclusive_pte);
> +                       ignore_exclusive_expected_pte =
> pte_swp_clear_exclusive(expected_pte);
> +
> +                       if (!pte_same(ignore_exclusive_pte,
> ignore_exclusive_expected_pte))
> +                               break;
> +               } else {
> +                       if (!pte_same(pte, expected_pte))
> +                               break;
> +               }
> 
>                 if (any_shared)
>                         *any_shared |= !pte_swp_exclusive(pte);

I'll leave David to comment on this proposal; I'm not sure I understand all the
details. The code change does look a bit "busy" though - sometimes that can be
an indicator :)

> 
>> softdirty and uffd-wp as well at least initially for simplicity.
> 
> yes for this.
> 
> By the way, I wonder if you and Ryan have a moment to review swpin
> refault patchset
> v2 :-)

It's on my todo list! I'm very keen to get as much large swap-out and swap-in
support into v6.10 as we can. Hoping to get to it inthe next couple of days.

> 
> [PATCH v2 0/5] large folios swap-in: handle refault cases first
> https://lore.kernel.org/linux-mm/20240409082631.187483-1-21cnbao@xxxxxxxxx/
> 
> 
>>
>> --
>> Cheers,
>>
>> David / dhildenb
>>
> 
> Thanks
> Barry