linux-kernel - [PATCH v2 2/4] mm/gup: handle ZONE_DEVICE pages in folio_fast_pin

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Date: Tue, 11 Jun 2024 12:27:30 -0600
From: Martin Oliveira <martin.oliveira@...eticom.com>
To: linux-rdma@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Cc: Jason Gunthorpe <jgg@...pe.ca>,
	Leon Romanovsky <leon@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Logan Gunthorpe <logang@...tatee.com>,
	Martin Oliveira <martin.oliveira@...eticom.com>,
	Mike Marciniszyn <mike.marciniszyn@...el.com>,
	Shiraz Saleem <shiraz.saleem@...el.com>,
	Michael Guralnik <michaelgur@...dia.com>,
	Artemy Kovalyov <artemyko@...dia.com>
Subject: [PATCH v2 2/4] mm/gup: handle ZONE_DEVICE pages in folio_fast_pin_allowed()

folio_fast_pin_allowed() does not support ZONE_DEVICE pages because
currently it is impossible for that type of page to be used with
FOLL_LONGTERM. When this changes in a subsequent patch, this path will
attempt to read the mapping of a ZONE_DEVICE page which is not valid.

Instead, allow ZONE_DEVICE pages explicitly seeing they shouldn't pose
any problem with the fast path.

Co-developed-by: Logan Gunthorpe <logang@...tatee.com>
Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
Signed-off-by: Martin Oliveira <martin.oliveira@...eticom.com>
---
 mm/gup.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index ca0f5cedce9b2..00d0a77112f4f 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2847,6 +2847,10 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
 	if (folio_test_hugetlb(folio))
 		return true;
 
+	/* It makes no sense to access the mapping of ZONE_DEVICE pages */
+	if (folio_is_zone_device(folio))
+		return true;
+
 	/*
 	 * GUP-fast disables IRQs. When IRQS are disabled, RCU grace periods
 	 * cannot proceed, which means no actions performed under RCU can
-- 
2.43.0