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

[John Hubbard <jhubbard@nvidia.com>]

On 9/5/25 11:56 PM, David Hildenbrand wrote:
> On 06.09.25 03:05, John Hubbard wrote:
>> On 9/1/25 8:03 AM, David Hildenbrand wrote:
...> Well, there is a lot I dislike about record_subpages() to go back 
there.
> Starting with "as Willy keeps explaining, the concept of subpages do
> not exist and ending with "why do we fill out the array even on failure".
> 
> :)

I am also very glad to see the entire concept of subpages disappear.

>>
>> Now it's been returned to it's original, cryptic form.
>>
> 
> The code in the caller was so uncryptic that both me and Lorenzo missed
> that magical addition. :P
> 
>> Just my take on it, for whatever that's worth. :)
> 
> As always, appreciated.
> 
> I could of course keep the simple loop in some "record_folio_pages"
> function and clean up what I dislike about record_subpages().
> 
> But I much rather want the call chain to be cleaned up instead, if 
> possible.
> 

Right! The primary way that record_subpages() helped was in showing
what was going on: a function call helps a lot to self-document,
sometimes.

> 
> Roughly, what I am thinking (limiting it to pte+pmd case) about is the 
> following:

The code below looks much cleaner, that's great!

thanks,
-- 
John Hubbard

> 
> 
>  From d6d6d21dbf435d8030782a627175e36e6c7b2dfb Mon Sep 17 00:00:00 2001
> From: David Hildenbrand <david@redhat.com>
> Date: Sat, 6 Sep 2025 08:33:42 +0200
> Subject: [PATCH] tmp
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>   mm/gup.c | 79 ++++++++++++++++++++++++++------------------------------
>   1 file changed, 36 insertions(+), 43 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 22420f2069ee1..98907ead749c0 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2845,12 +2845,11 @@ static void __maybe_unused 
> gup_fast_undo_dev_pagemap(int *nr, int nr_start,
>    * also check pmd here to make sure pmd doesn't change (corresponds to
>    * pmdp_collapse_flush() in the THP collapse code path).
>    */
> -static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
> -        unsigned long end, unsigned int flags, struct page **pages,
> -        int *nr)
> +static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, 
> unsigned long addr,
> +        unsigned long end, unsigned int flags, struct page **pages)
>   {
>       struct dev_pagemap *pgmap = NULL;
> -    int ret = 0;
> +    unsigned long nr_pages = 0;
>       pte_t *ptep, *ptem;
> 
>       ptem = ptep = pte_offset_map(&pmd, addr);
> @@ -2908,24 +2907,20 @@ static int gup_fast_pte_range(pmd_t pmd, pmd_t 
> *pmdp, unsigned long addr,
>            * details.
>            */
>           if (flags & FOLL_PIN) {
> -            ret = arch_make_folio_accessible(folio);
> -            if (ret) {
> +            if (arch_make_folio_accessible(folio)) {
>                   gup_put_folio(folio, 1, flags);
>                   goto pte_unmap;
>               }
>           }
>           folio_set_referenced(folio);
> -        pages[*nr] = page;
> -        (*nr)++;
> +        pages[nr_pages++] = page;
>       } while (ptep++, addr += PAGE_SIZE, addr != end);
> 
> -    ret = 1;
> -
>   pte_unmap:
>       if (pgmap)
>           put_dev_pagemap(pgmap);
>       pte_unmap(ptem);
> -    return ret;
> +    return nr_pages;
>   }
>   #else
> 
> @@ -2938,21 +2933,24 @@ static int gup_fast_pte_range(pmd_t pmd, pmd_t 
> *pmdp, unsigned long addr,
>    * get_user_pages_fast_only implementation that can pin pages. Thus 
> it's still
>    * useful to have gup_fast_pmd_leaf even if we can't operate on ptes.
>    */
> -static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
> -        unsigned long end, unsigned int flags, struct page **pages,
> -        int *nr)
> +static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, 
> unsigned long addr,
> +        unsigned long end, unsigned int flags, struct page **pages)
>   {
>       return 0;
>   }
>   #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */
> 
> -static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr,
> -        unsigned long end, unsigned int flags, struct page **pages,
> -        int *nr)
> +static unsigned long gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, 
> unsigned long addr,
> +        unsigned long end, unsigned int flags, struct page **pages)
>   {
> +    const unsigned long nr_pages = (end - addr) >> PAGE_SHIFT;
>       struct page *page;
>       struct folio *folio;
> -    int refs;
> +    unsigned long i;
> +
> +    /* See gup_fast_pte_range() */
> +    if (pmd_protnone(orig))
> +        return 0;
> 
>       if (!pmd_access_permitted(orig, flags & FOLL_WRITE))
>           return 0;
> @@ -2960,33 +2958,30 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t 
> *pmdp, unsigned long addr,
>       if (pmd_special(orig))
>           return 0;
> 
> -    refs = (end - addr) >> PAGE_SHIFT;
>       page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
> 
> -    folio = try_grab_folio_fast(page, refs, flags);
> +    folio = try_grab_folio_fast(page, nr_pages, flags);
>       if (!folio)
>           return 0;
> 
>       if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
> -        gup_put_folio(folio, refs, flags);
> +        gup_put_folio(folio, nr_pages, flags);
>           return 0;
>       }
> 
>       if (!gup_fast_folio_allowed(folio, flags)) {
> -        gup_put_folio(folio, refs, flags);
> +        gup_put_folio(folio, nr_pages, flags);
>           return 0;
>       }
>       if (!pmd_write(orig) && gup_must_unshare(NULL, flags, &folio- 
>  >page)) {
> -        gup_put_folio(folio, refs, flags);
> +        gup_put_folio(folio, nr_pages, flags);
>           return 0;
>       }
> 
> -    pages += *nr;
> -    *nr += refs;
> -    for (; refs; refs--)
> +    for (i = 0; i < nr_pages; i++)
>           *(pages++) = page++;
>       folio_set_referenced(folio);
> -    return 1;
> +    return nr_pages;
>   }
> 
>   static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr,
> @@ -3033,11 +3028,11 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t 
> *pudp, unsigned long addr,
>       return 1;
>   }
> 
> -static int gup_fast_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr,
> -        unsigned long end, unsigned int flags, struct page **pages,
> -        int *nr)
> +static unsigned long gup_fast_pmd_range(pud_t *pudp, pud_t pud, 
> unsigned long addr,
> +        unsigned long end, unsigned int flags, struct page **pages)
>   {
> -    unsigned long next;
> +    unsigned long cur_nr_pages, next;
> +    unsigned long nr_pages = 0;
>       pmd_t *pmdp;
> 
>       pmdp = pmd_offset_lockless(pudp, pud, addr);
> @@ -3046,23 +3041,21 @@ static int gup_fast_pmd_range(pud_t *pudp, pud_t 
> pud, unsigned long addr,
> 
>           next = pmd_addr_end(addr, end);
>           if (!pmd_present(pmd))
> -            return 0;
> +            break;
> 
> -        if (unlikely(pmd_leaf(pmd))) {
> -            /* See gup_fast_pte_range() */
> -            if (pmd_protnone(pmd))
> -                return 0;
> +        if (unlikely(pmd_leaf(pmd)))
> +            cur_nr_pages = gup_fast_pmd_leaf(pmd, pmdp, addr, next, 
> flags, pages);
> +        else
> +            cur_nr_pages = gup_fast_pte_range(pmd, pmdp, addr, next, 
> flags, pages);
> 
> -            if (!gup_fast_pmd_leaf(pmd, pmdp, addr, next, flags,
> -                pages, nr))
> -                return 0;
> +        nr_pages += cur_nr_pages;
> +        pages += cur_nr_pages;
> 
> -        } else if (!gup_fast_pte_range(pmd, pmdp, addr, next, flags,
> -                           pages, nr))
> -            return 0;
> +        if (nr_pages != (next - addr) >> PAGE_SIZE)
> +            break;
>       } while (pmdp++, addr = next, addr != end);
> 
> -    return 1;
> +    return nr_pages;
>   }
> 
>   static int gup_fast_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr,