Re: [PATCH] mm/gup: fix GUP-fast fallback for NULL-mapping order-0 folios

John Hubbard
2026-04-09T02:09:44Z

On 4/8/26 7:07 PM, Andrew Morton wrote:
> On Wed, 8 Apr 2026 18:46:47 -0700 John Hubbard <jhubbard@nvidia.com> wrote:
>
>> Since commit f002882ca369 ("mm: merge folio_is_secretmem() and
>> folio_fast_pin_allowed() into gup_fast_folio_allowed()"),
>> gup_fast_folio_allowed() falls back to the slow path for any order-0
>> folio with a NULL mapping when CONFIG_SECRETMEM=y. This causes a
>> performance regression for drivers that allocate pages with alloc_page()
>> and insert them into VMAs via vm_insert_page(). These pages legitimately
>> have a NULL folio->mapping, but they cannot be secretmem pages.
>
> How significant is the slowdown?

About 2x!

Sourab has a bunch of peer to peer IO cases that use 4K pages,
that he ran.

thanks,
--
John Hubbard