[PATCH v2 19/37] mm/gup: remove record_subpages()

[Jens Axboe]

On 9/5/25 12:41 AM, David Hildenbrand wrote:
> On 01.09.25 17:03, David Hildenbrand wrote:
>> We can just cleanup the code by calculating the #refs earlier,
>> so we can just inline what remains of record_subpages().
>>
>> Calculate the number of references/pages ahead of times, and record them
>> only once all our tests passed.
>>
>> Signed-off-by: David Hildenbrand <david at redhat.com>
>> ---
>>   mm/gup.c | 25 ++++++++-----------------
>>   1 file changed, 8 insertions(+), 17 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index c10cd969c1a3b..f0f4d1a68e094 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -484,19 +484,6 @@ static inline void mm_set_has_pinned_flag(struct mm_struct *mm)
>>   #ifdef CONFIG_MMU
>>     #ifdef CONFIG_HAVE_GUP_FAST
>> -static int record_subpages(struct page *page, unsigned long sz,
>> -               unsigned long addr, unsigned long end,
>> -               struct page **pages)
>> -{
>> -    int nr;
>> -
>> -    page += (addr & (sz - 1)) >> PAGE_SHIFT;
>> -    for (nr = 0; addr != end; nr++, addr += PAGE_SIZE)
>> -        pages[nr] = page++;
>> -
>> -    return nr;
>> -}
>> -
>>   /**
>>    * try_grab_folio_fast() - Attempt to get or pin a folio in fast path.
>>    * @page:  pointer to page to be grabbed
>> @@ -2967,8 +2954,8 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr,
>>       if (pmd_special(orig))
>>           return 0;
>>   -    page = pmd_page(orig);
>> -    refs = record_subpages(page, PMD_SIZE, addr, end, pages + *nr);
>> +    refs = (end - addr) >> PAGE_SHIFT;
>> +    page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
>>         folio = try_grab_folio_fast(page, refs, flags);
>>       if (!folio)
>> @@ -2989,6 +2976,8 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr,
>>       }
>>         *nr += refs;
>> +    for (; refs; refs--)
>> +        *(pages++) = page++;
>>       folio_set_referenced(folio);
>>       return 1;
>>   }
>> @@ -3007,8 +2996,8 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr,
>>       if (pud_special(orig))
>>           return 0;
>>   -    page = pud_page(orig);
>> -    refs = record_subpages(page, PUD_SIZE, addr, end, pages + *nr);
>> +    refs = (end - addr) >> PAGE_SHIFT;
>> +    page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
>>         folio = try_grab_folio_fast(page, refs, flags);
>>       if (!folio)
>> @@ -3030,6 +3019,8 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr,
>>       }
>>         *nr += refs;
>> +    for (; refs; refs--)
>> +        *(pages++) = page++;
>>       folio_set_referenced(folio);
>>       return 1;
>>   }
> 
> Okay, this code is nasty. We should rework this code to just return the nr and receive a the proper
> pages pointer, getting rid of the "*nr" parameter.
> 
> For the time being, the following should do the trick:
> 
> commit bfd07c995814354f6b66c5b6a72e96a7aa9fb73b (HEAD -> nth_page)
> Author: David Hildenbrand <david at redhat.com>
> Date:   Fri Sep 5 08:38:43 2025 +0200
> 
>     fixup: mm/gup: remove record_subpages()
>         pages is not adjusted by the caller, but idnexed by existing *nr.
>         Signed-off-by: David Hildenbrand <david at redhat.com>
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 010fe56f6e132..22420f2069ee1 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2981,6 +2981,7 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr,
>                 return 0;
>         }
>  
> +       pages += *nr;
>         *nr += refs;
>         for (; refs; refs--)
>                 *(pages++) = page++;
> @@ -3024,6 +3025,7 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr,
>                 return 0;
>         }
>  
> +       pages += *nr;
>         *nr += refs;
>         for (; refs; refs--)
>                 *(pages++) = page++;
> 

Tested as fixing the issue for me, thanks.

-- 
Jens Axboe