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

Eric Biggers
2025-09-05T23:01:26Z

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.

Thanks,

- Eric