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Message-ID: <1513d5fd-14ef-4cd0-a9a5-1016e9be6540@kernel.dk>
Date: Fri, 5 Sep 2025 05:26:53 -0600
From: Jens Axboe <axboe@...nel.dk>
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>, 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, 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 9/5/25 12:41 AM, 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@...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++;
>> 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@...hat.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@...hat.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++;
>
Tested as fixing the issue for me, thanks.
--
Jens Axboe
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