Date: Tue, 27 Feb 2024 10:36:42 +0100
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>
Cc: Ryan Roberts <ryan.roberts@....com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Barry Song <v-songbaohua@...o.com>, Lance Yang <ioworker0@...il.com>,
 Yin Fengwei <fengwei.yin@...el.com>
Subject: Re: [PATCH] mm: export folio_pte_batch as a couple of modules might
 need it

On 27.02.24 10:27, Barry Song wrote:
> On Tue, Feb 27, 2024 at 10:14 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 27.02.24 10:07, Ryan Roberts wrote:
>>> On 27/02/2024 02:40, Barry Song wrote:
>>>> From: Barry Song <v-songbaohua@...o.com>
>>>>
>>>> madvise and some others might need folio_pte_batch to check if a range
>>>> of PTEs are completely mapped to a large folio with contiguous physcial
>>>> addresses. Let's export it for others to use.
>>>>
>>>> Cc: Lance Yang <ioworker0@...il.com>
>>>> Cc: Ryan Roberts <ryan.roberts@....com>
>>>> Cc: David Hildenbrand <david@...hat.com>
>>>> Cc: Yin Fengwei <fengwei.yin@...el.com>
>>>> Signed-off-by: Barry Song <v-songbaohua@...o.com>
>>>> ---
>>>>    -v1:
>>>>    at least two jobs madv_free and madv_pageout depend on it. To avoid
>>>>    conflicts and dependencies, after discussing with Lance, we prefer
>>>>    this one can land earlier.
>>>
>>> I think this will also ultimately be useful for mprotect too, though I haven't
>>> looked at it properly yet.
>>>
>>
>> Yes, I think we briefly discussed that.
>>
>>>>
>>>>    mm/internal.h | 13 +++++++++++++
>>>>    mm/memory.c   | 11 +----------
>>>>    2 files changed, 14 insertions(+), 10 deletions(-)
>>>>
>>>> diff --git a/mm/internal.h b/mm/internal.h
>>>> index 13b59d384845..8e2bc304f671 100644
>>>> --- a/mm/internal.h
>>>> +++ b/mm/internal.h
>>>> @@ -83,6 +83,19 @@ static inline void *folio_raw_mapping(struct folio *folio)
>>>>       return (void *)(mapping & ~PAGE_MAPPING_FLAGS);
>>>>    }
>>>>
>>>> +/* Flags for folio_pte_batch(). */
>>>> +typedef int __bitwise fpb_t;
>>>> +
>>>> +/* Compare PTEs after pte_mkclean(), ignoring the dirty bit. */
>>>> +#define FPB_IGNORE_DIRTY            ((__force fpb_t)BIT(0))
>>>> +
>>>> +/* Compare PTEs after pte_clear_soft_dirty(), ignoring the soft-dirty bit. */
>>>> +#define FPB_IGNORE_SOFT_DIRTY               ((__force fpb_t)BIT(1))
>>>> +
>>>> +extern int folio_pte_batch(struct folio *folio, unsigned long addr,
>>>> +            pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
>>>> +            bool *any_writable);
>>>> +
>>>>    void __acct_reclaim_writeback(pg_data_t *pgdat, struct folio *folio,
>>>>                                               int nr_throttled);
>>>>    static inline void acct_reclaim_writeback(struct folio *folio)
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index 1c45b6a42a1b..319b3be05e75 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -953,15 +953,6 @@ static __always_inline void __copy_present_ptes(struct vm_area_struct *dst_vma,
>>>>       set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr);
>>>>    }
>>>>
>>>> -/* Flags for folio_pte_batch(). */
>>>> -typedef int __bitwise fpb_t;
>>>> -
>>>> -/* Compare PTEs after pte_mkclean(), ignoring the dirty bit. */
>>>> -#define FPB_IGNORE_DIRTY            ((__force fpb_t)BIT(0))
>>>> -
>>>> -/* Compare PTEs after pte_clear_soft_dirty(), ignoring the soft-dirty bit. */
>>>> -#define FPB_IGNORE_SOFT_DIRTY               ((__force fpb_t)BIT(1))
>>>> -
>>>>    static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>>>>    {
>>>>       if (flags & FPB_IGNORE_DIRTY)
>>>> @@ -982,7 +973,7 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>>>>     * If "any_writable" is set, it will indicate if any other PTE besides the
>>>>     * first (given) PTE is writable.
>>>>     */
>>>
>>> David was talking in Lance's patch thread, about improving the docs for this
>>> function now that its exported. Might be worth syncing on that.
>>
>> Here is my take:
>>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>>    mm/memory.c | 22 ++++++++++++++++++----
>>    1 file changed, 18 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index d0b855a1837a8..098356b8805ae 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -971,16 +971,28 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
>>          return pte_wrprotect(pte_mkold(pte));
>>    }
>>
>> -/*
>> +/**
>> + * folio_pte_batch - detect a PTE batch for a large folio
>> + * @folio: The large folio to detect a PTE batch for.
>> + * @addr: The user virtual address the first page is mapped at.
>> + * @start_ptep: Page table pointer for the first entry.
>> + * @pte: Page table entry for the first page.
>> + * @max_nr: The maximum number of table entries to consider.
>> + * @flags: Flags to modify the PTE batch semantics.
>> + * @any_writable: Optional pointer to indicate whether any entry except the
>> + *               first one is writable.
>> + *
>>     * Detect a PTE batch: consecutive (present) PTEs that map consecutive
>> - * pages of the same folio.
>> + * pages of the same large folio.
>>     *
>>     * All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN,
>>     * the accessed bit, writable bit, dirty bit (with FPB_IGNORE_DIRTY) and
>>     * soft-dirty bit (with FPB_IGNORE_SOFT_DIRTY).
>>     *
>> - * If "any_writable" is set, it will indicate if any other PTE besides the
>> - * first (given) PTE is writable.
>> + * start_ptep must map any page of the folio. 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 folio_pte_batch(struct folio *folio, unsigned long addr,
>>                  pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
>> @@ -996,6 +1008,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>>                  *any_writable = false;
>>
>>          VM_WARN_ON_FOLIO(!pte_present(pte), folio);
>> +       VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio);
>> +       VM_WARN_ON_FOLIO(page_folio(pfn_to_page(pte_pfn(pte))) != folio, folio);
>>
>>          nr = pte_batch_hint(start_ptep, pte);
>>          expected_pte = __pte_batch_clear_ignored(pte_advance_pfn(pte, nr), flags);
>> --
>> 2.43.2
>>
>>
>>>
>>>> -static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>>>> +int folio_pte_batch(struct folio *folio, unsigned long addr,
>>>
>>> fork() is very performance sensitive. Is there a risk we are regressing
>>> performance by making this out-of-line? Although its in the same compilation
>>> unit so the compiler may well inline it anyway?
>>
>> Easy to verify by looking at the generated asm I guess?
> 
> my aarch64-linux-gnu-gcc didn't inline it

I think on x86-64 it would inline it with "gcc (GCC) 13.2.1 20231205 
(Red Hat 13.2.1-6)"

> 
> $ aarch64-linux-gnu-gcc --version
> aarch64-linux-gnu-gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
> Copyright (C) 2021 Free Software Foundation, Inc.
> 
> $ nm -S -s vmlinux.a | grep  folio_pte_batch
> 0000000000003818 0000000000000204 T folio_pte_batch
> 

As it's only used on the folio_test_large() "slower" paths, likely 
optimizing out the "writable" check (and possibly the flags) might not 
be that important.

>>
>>>
>>> Either way, perhaps we are better off making it inline in the header? That would
>>> avoid needing to rerun David's micro-benchmarks for fork() and munmap().
> 
> actually tried this before trying extern, the problem is that we have to add
> others into internal.h, for example __pte_batch_clear_ignored, which
> seems not API. are we comfortable to move that one to internal.h too?

Yes, that shouldn't stop us.

-- 
Cheers,

David / dhildenb

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