Re: [PATCH v1 18/36] mm/gup: drop nth_page() usage within folio when recording subpages

On 28.08.25 18:37, Lorenzo Stoakes wrote:
> On Thu, Aug 28, 2025 at 12:01:22AM +0200, David Hildenbrand wrote:
>> nth_page() is no longer required when iterating over pages within a
>> single folio, so let's just drop it when recording subpages.
>>
>> Signed-off-by: David Hildenbrand <[email protected]>
> 
> This looks correct to me, so notwithtsanding suggestion below, LGTM and:
> 
> Reviewed-by: Lorenzo Stoakes <[email protected]>
> 
>> ---
>>   mm/gup.c | 7 +++----
>>   1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index b2a78f0291273..89ca0813791ab 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -488,12 +488,11 @@ static int record_subpages(struct page *page, unsigned long sz,
>>   			   unsigned long addr, unsigned long end,
>>   			   struct page **pages)
>>   {
>> -	struct page *start_page;
>>   	int nr;
>>
>> -	start_page = nth_page(page, (addr & (sz - 1)) >> PAGE_SHIFT);
>> +	page += (addr & (sz - 1)) >> PAGE_SHIFT;
>>   	for (nr = 0; addr != end; nr++, addr += PAGE_SIZE)
>> -		pages[nr] = nth_page(start_page, nr);
>> +		pages[nr] = page++;
> 
> 
> This is really nice, but I wonder if (while we're here) we can't be even
> more clear as to what's going on here, e.g.:
> 
> static int record_subpages(struct page *page, unsigned long sz,
> 			   unsigned long addr, unsigned long end,
> 			   struct page **pages)
> {
> 	size_t offset_in_folio = (addr & (sz - 1)) >> PAGE_SHIFT;
> 	struct page *subpage = page + offset_in_folio;
> 
> 	for (; addr != end; addr += PAGE_SIZE)
> 		*pages++ = subpage++;
> 
> 	return nr;
> }
> 
> Or some variant of that with the masking stuff self-documented.

What about the following cleanup on top:


diff --git a/mm/gup.c b/mm/gup.c
index 89ca0813791ab..5a72a135ec70b 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
@@ -2963,8 +2950,8 @@ 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)
@@ -2985,6 +2972,8 @@ 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;
  }
@@ -3003,8 +2992,8 @@ 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)
@@ -3026,6 +3015,8 @@ 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;
  }


The nice thing is that we only record pages in the array if they actually passed our tests.


-- 
Cheers

David / dhildenb