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

On 05.09.25 13:34, Lorenzo Stoakes 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 <[email protected]>
> 
> So strange I thought I looked at this...!
> 
>>> ---
>>>    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 <[email protected]>
>> 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 <[email protected]>
>>
>> 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++;
> 
> This looks correct.
> 
> But.
> 
> This is VERY nasty. Before we'd call record_subpages() with pages + *nr, where
> it was clear we were offsetting by this, now we're making things imo way more
> confusing.
> 
> This makes me less in love with this approach to be honest.
> 
> But perhaps it's the least worst thing for now until we can do a bigger
> refactor...
> 
> So since this seems correct to me, and for the sake of moving things forward
> (was this one patch dropped from mm-new or does mm-new just have an old version?
> Confused):
> 
> Reviewed-by: Lorenzo Stoakes <[email protected]>
> 
> For this patch obviously with the fix applied.
> 
> But can we PLEASE revisit this :)

Yeah, I already asked someone internally if he would have time to do 
some refactorings in mm/gup.c.

If that won't work out I shall do it at some point (and the same time 
reworking follow_page_mask() to just consume the array as well like gup 
does)

-- 
Cheers

David / dhildenb