netdev - Re: [PATCH v2 19/37] mm/gup: remove record

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Message-ID: <016307ba-427d-4646-8e4d-1ffefd2c1968@nvidia.com>
Date: Fri, 5 Sep 2025 18:05:21 -0700
From: John Hubbard <jhubbard@...dia.com>
To: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc: Alexander Potapenko <glider@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Brendan Jackman <jackmanb@...gle.com>, Christoph Lameter <cl@...two.org>,
 Dennis Zhou <dennis@...nel.org>, Dmitry Vyukov <dvyukov@...gle.com>,
 dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
 iommu@...ts.linux.dev, io-uring@...r.kernel.org,
 Jason Gunthorpe <jgg@...dia.com>, Jens Axboe <axboe@...nel.dk>,
 Johannes Weiner <hannes@...xchg.org>, kasan-dev@...glegroups.com,
 kvm@...r.kernel.org, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-arm-kernel@...s.com,
 linux-arm-kernel@...ts.infradead.org, linux-crypto@...r.kernel.org,
 linux-ide@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-mips@...r.kernel.org, linux-mmc@...r.kernel.org, linux-mm@...ck.org,
 linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
 linux-scsi@...r.kernel.org, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Marco Elver <elver@...gle.com>, Marek Szyprowski <m.szyprowski@...sung.com>,
 Michal Hocko <mhocko@...e.com>, Mike Rapoport <rppt@...nel.org>,
 Muchun Song <muchun.song@...ux.dev>, netdev@...r.kernel.org,
 Oscar Salvador <osalvador@...e.de>, Peter Xu <peterx@...hat.com>,
 Robin Murphy <robin.murphy@....com>, Suren Baghdasaryan <surenb@...gle.com>,
 Tejun Heo <tj@...nel.org>, virtualization@...ts.linux.dev,
 Vlastimil Babka <vbabka@...e.cz>, wireguard@...ts.zx2c4.com, x86@...nel.org,
 Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH v2 19/37] mm/gup: remove record_subpages()

On 9/1/25 8:03 AM, 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@...hat.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++;

Hi David,

Probably a similar sentiment as Lorenzo here...the above diffs make the code
*worse* to read. In fact, I recall adding record_subpages() here long ago,
specifically to help clarify what was going on.

Now it's been returned to it's original, cryptic form.

Just my take on it, for whatever that's worth. :)

thanks,
-- 
John Hubbard

>  	folio_set_referenced(folio);
>  	return 1;
>  }