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

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Message-ID: <44072455-fc68-430d-ad38-0b9ce6a10b8d@redhat.com>
Date: Mon, 1 Sep 2025 13:35:05 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, 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>, John Hubbard <jhubbard@...dia.com>,
 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, 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 v1 18/36] mm/gup: drop nth_page() usage within folio when
 recording subpages

>>
>>
>> The nice thing is that we only record pages in the array if they actually passed our tests.
> 
> Yeah that's nice actually.
> 
> This is fine (not the meme :P)

:D

> 
> So yes let's do this!

That leaves us with the following on top of this patch:

 From 4533c6e3590cab0c53e81045624d5949e0ad9015 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@...hat.com>
Date: Fri, 29 Aug 2025 15:41:45 +0200
Subject: [PATCH] mm/gup: remove record_subpages()

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 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;
  }
-- 
2.50.1

-- 
Cheers

David / dhildenb