linux-kernel - Re: [PATCH] mm: export folio_pte_batch as a couple of modules might need it

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Message-ID: <dbce43b4-cf69-41d0-af6c-fdfc3032fedb@redhat.com>
Date: Tue, 27 Feb 2024 11:30:14 +0100
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>
Cc: Ryan Roberts <ryan.roberts@....com>, Barry Song <21cnbao@...il.com>,
 akpm@...ux-foundation.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Barry Song <v-songbaohua@...o.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 11:21, Lance Yang wrote:
> On Tue, Feb 27, 2024 at 5:14 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.
> 
> Nit:
> 
> - * @pte: Page table entry for the first page.
> + * @pte: Page table entry for the first page that must be the first subpage of
> + *               the folio excluding arm64 for now.
> 
> IIUC, pte_batch_hint is always 1 excluding arm64 for now.
> I'm not sure if this modification will be helpful?

IIRC, Ryan made sure that this also works when passing another subpage, 
after when cont-pte is set. Otherwise this would already be broken for 
fork/zap.

So I don't think this comment would actually be correct.

-- 
Cheers,

David / dhildenb