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

Jens Axboe
2025-09-05T11:26:56Z

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@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@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@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