Re: [PATCH v6 2/6] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()

[Ryan Roberts <ryan.roberts@arm.com>]

On 08/04/2024 13:47, Ryan Roberts wrote:
> On 08/04/2024 13:07, Ryan Roberts wrote:
>> [...]
>>>
>>> [...]
>>>
>>>> +
>>>> +/**
>>>> + * swap_pte_batch - detect a PTE batch for a set of contiguous swap entries
>>>> + * @start_ptep: Page table pointer for the first entry.
>>>> + * @max_nr: The maximum number of table entries to consider.
>>>> + * @entry: Swap entry recovered from the first table entry.
>>>> + *
>>>> + * Detect a batch of contiguous swap entries: consecutive (non-present) PTEs
>>>> + * containing swap entries all with consecutive offsets and targeting the same
>>>> + * swap type.
>>>> + *
>>>
>>> Likely you should document that any swp pte bits are ignored? ()
>>
>> Now that I understand what swp pte bits are, I think the simplest thing is to
>> just make this function always consider the pte bits by using pte_same() as you
>> suggest below? I don't think there is ever a case for ignoring the swp pte bits?
>> And then I don't need to do anything special for uffd-wp either (below you
>> suggested not doing batching when the VMA has uffd enabled).
>>
>> Any concerns?
>>
>>>
>>>> + * max_nr must be at least one and must be limited by the caller so scanning
>>>> + * cannot exceed a single page table.
>>>> + *
>>>> + * Return: the number of table entries in the batch.
>>>> + */
>>>> +static inline int swap_pte_batch(pte_t *start_ptep, int max_nr,
>>>> +                 swp_entry_t entry)
>>>> +{
>>>> +    const pte_t *end_ptep = start_ptep + max_nr;
>>>> +    unsigned long expected_offset = swp_offset(entry) + 1;
>>>> +    unsigned int expected_type = swp_type(entry);
>>>> +    pte_t *ptep = start_ptep + 1;
>>>> +
>>>> +    VM_WARN_ON(max_nr < 1);
>>>> +    VM_WARN_ON(non_swap_entry(entry));
>>>> +
>>>> +    while (ptep < end_ptep) {
>>>> +        pte_t pte = ptep_get(ptep);
>>>> +
>>>> +        if (pte_none(pte) || pte_present(pte))
>>>> +            break;
>>>> +
>>>> +        entry = pte_to_swp_entry(pte);
>>>> +
>>>> +        if (non_swap_entry(entry) ||
>>>> +            swp_type(entry) != expected_type ||
>>>> +            swp_offset(entry) != expected_offset)
>>>> +            break;
>>>> +
>>>> +        expected_offset++;
>>>> +        ptep++;
>>>> +    }
>>>> +
>>>> +    return ptep - start_ptep;
>>>> +}
>>>
>>> Looks very clean :)
>>>
>>> I was wondering whether we could similarly construct the expected swp PTE and
>>> only check pte_same.
>>>
>>> expected_pte = __swp_entry_to_pte(__swp_entry(expected_type, expected_offset));
>>
>> So planning to do this.
> 
> Of course this clears all the swp pte bits in expected_pte. So need to do something a bit more complex.
> 
> If we can safely assume all offset bits are contiguous in every per-arch representation then we can do:

Looks like at least csky and hexagon store the offset in discontiguous regions.
So it will have to be the second approach if we want to avoid anything
arch-specific. I'll assume that for now; we can always specialize
pte_next_swp_offset() per-arch in the future if needed.

> 
> static inline pte_t pte_next_swp_offset(pte_t pte)
> {
> 	pte_t offset_inc = __swp_entry_to_pte(__swp_entry(0, 1));
> 
> 	return __pte(pte_val(pte) + pte_val(offset_inc));
> }
> 
> Or if not:
> 
> 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);
> 
> 	return new;
> }
> 
> Then swap_pte_batch() becomes:
> 
> static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, pte_t pte)
> {
> 	pte_t expected_pte = pte_next_swp_offset(pte);
> 	const pte_t *end_ptep = start_ptep + max_nr;
> 	pte_t *ptep = start_ptep + 1;
> 
> 	VM_WARN_ON(max_nr < 1);
> 	VM_WARN_ON(!is_swap_pte(pte));
> 	VM_WARN_ON(non_swap_entry(pte_to_swp_entry(pte)));
> 
> 	while (ptep < end_ptep) {
> 		pte = ptep_get(ptep);
> 
> 		if (!pte_same(pte, expected_pte))
> 			break;
> 
> 		expected_pte = pte_next_swp_offset(expected_pte);
> 		ptep++;
> 	}
> 
> 	return ptep - start_ptep;
> }
> 
> Would you be happy with either of these? I'll go look if we can assume the offset bits are always contiguous.
> 
> 
>>
>>>
>>> ... or have a variant to increase only the swp offset for an existing pte. But
>>> non-trivial due to the arch-dependent format.
>>
>> not this - I agree this will be difficult due to per-arch changes. I'd rather
>> just do the generic version and leave the compiler to do the best it can to
>> simplify and optimize.
>>
>>>
>>> But then, we'd fail on mismatch of other swp pte bits.
>>>
>>>
>>> On swapin, when reusing this function (likely!), we'll might to make sure that
>>> the PTE bits match as well.
>>>
>>> See below regarding uffd-wp.
>>>
>>>
>>>>   #endif /* CONFIG_MMU */
>>>>     void __acct_reclaim_writeback(pg_data_t *pgdat, struct folio *folio,
>>>> diff --git a/mm/madvise.c b/mm/madvise.c
>>>> index 1f77a51baaac..070bedb4996e 100644
>>>> --- a/mm/madvise.c
>>>> +++ b/mm/madvise.c
>>>> @@ -628,6 +628,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned
>>>> long addr,
>>>>       struct folio *folio;
>>>>       int nr_swap = 0;
>>>>       unsigned long next;
>>>> +    int nr, max_nr;
>>>>         next = pmd_addr_end(addr, end);
>>>>       if (pmd_trans_huge(*pmd))
>>>> @@ -640,7 +641,8 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned
>>>> long addr,
>>>>           return 0;
>>>>       flush_tlb_batched_pending(mm);
>>>>       arch_enter_lazy_mmu_mode();
>>>> -    for (; addr != end; pte++, addr += PAGE_SIZE) {
>>>> +    for (; addr != end; pte += nr, addr += PAGE_SIZE * nr) {
>>>> +        nr = 1;
>>>>           ptent = ptep_get(pte);
>>>>             if (pte_none(ptent))
>>>> @@ -655,9 +657,11 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned
>>>> long addr,
>>>>                 entry = pte_to_swp_entry(ptent);
>>>>               if (!non_swap_entry(entry)) {
>>>> -                nr_swap--;
>>>> -                free_swap_and_cache(entry);
>>>> -                pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
>>>> +                max_nr = (end - addr) / PAGE_SIZE;
>>>> +                nr = swap_pte_batch(pte, max_nr, entry);
>>>> +                nr_swap -= nr;
>>>> +                free_swap_and_cache_nr(entry, nr);
>>>> +                clear_not_present_full_ptes(mm, addr, pte, nr, tlb->fullmm);
>>>>               } else if (is_hwpoison_entry(entry) ||
>>>>                      is_poisoned_swp_entry(entry)) {
>>>>                   pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index 7dc6c3d9fa83..ef2968894718 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -1637,12 +1637,13 @@ static unsigned long zap_pte_range(struct mmu_gather
>>>> *tlb,
>>>>                   folio_remove_rmap_pte(folio, page, vma);
>>>>               folio_put(folio);
>>>>           } else if (!non_swap_entry(entry)) {
>>>> -            /* Genuine swap entry, hence a private anon page */
>>>> +            max_nr = (end - addr) / PAGE_SIZE;
>>>> +            nr = swap_pte_batch(pte, max_nr, entry);
>>>> +            /* Genuine swap entries, hence a private anon pages */
>>>>               if (!should_zap_cows(details))
>>>>                   continue;
>>>> -            rss[MM_SWAPENTS]--;
>>>> -            if (unlikely(!free_swap_and_cache(entry)))
>>>> -                print_bad_pte(vma, addr, ptent, NULL);
>>>> +            rss[MM_SWAPENTS] -= nr;
>>>> +            free_swap_and_cache_nr(entry, nr);
>>>>           } else if (is_migration_entry(entry)) {
>>>>               folio = pfn_swap_entry_folio(entry);
>>>>               if (!should_zap_folio(details, folio))
>>>> @@ -1665,8 +1666,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>>>>               pr_alert("unrecognized swap entry 0x%lx\n", entry.val);
>>>>               WARN_ON_ONCE(1);
>>>>           }
>>>> -        pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
>>>> -        zap_install_uffd_wp_if_needed(vma, addr, pte, 1, details, ptent);
>>>> +        clear_not_present_full_ptes(mm, addr, pte, nr, tlb->fullmm);
>>>
>>> For zap_install_uffd_wp_if_needed(), the uffd-wp bit has to match.
>>>
>>> zap_install_uffd_wp_if_needed() will use the uffd-wp information in
>>> ptent->pteval to make a decision whether to place PTE_MARKER_UFFD_WP markers.
>>>
>>> On mixture, you either lose some or place too many markers.
>>>
>>> A simple workaround would be to disable any such batching if the VMA does have
>>> uffd-wp enabled.
>>
>> Rather than this, I'll just consider all the swp pte bits when batching.
>>
>>>
>>>> +        zap_install_uffd_wp_if_needed(vma, addr, pte, nr, details, ptent);
>>>>       } while (pte += nr, addr += PAGE_SIZE * nr, addr != end);
>>
>> [...]
>>
>