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

David Hildenbrand
2025-09-06T06:59:11Z

On 06.09.25 01:00, Eric Biggers wrote:
> On Fri, Sep 05, 2025 at 08:41:23AM +0200, 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++;
>
> Can this get folded in soon? This bug is causing crashes in AF_ALG too.

Andrew immediately dropped the original patch, so it's gone from
mm-unstable and should be gone from next soon (today?).

--
Cheers

David / dhildenb