linux-kernel - [PATCH v1 2/3] mm/gup: use gup_can_follow

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Date:   Thu, 25 Aug 2022 18:46:58 +0200
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, Peter Xu <peterx@...hat.com>
Subject: [PATCH v1 2/3] mm/gup: use gup_can_follow_protnone() also in GUP-fast

There seems to be no reason why FOLL_FORCE during GUP-fast would have to
fallback to the slow path when stumbling over a PROT_NONE mapped page. We
only have to trigger hinting faults in case FOLL_FORCE is not set, and any
kind of fault handling naturally happens from the slow path -- where
NUMA hinting accounting/handling would be performed.

Note that the comment regarding THP migration is outdated:
commit 2b4847e73004 ("mm: numa: serialise parallel get_user_page against
THP migration") described that this was required for THP due to lack of PMD
migration entries. Nowadays, we do have proper PMD migration entries in
place -- see set_pmd_migration_entry(), which does a proper
pmdp_invalidate() when placing the migration entry.

So let's just reuse gup_can_follow_protnone() here to make it
consistent and drop the somewhat outdated comments.

Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/gup.c | 14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index a1355dbd848e..dfef23071dc8 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2350,11 +2350,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
 		struct page *page;
 		struct folio *folio;

-		/*
-		 * Similar to the PMD case below, NUMA hinting must take slow
-		 * path using the pte_protnone check.
-		 */
-		if (pte_protnone(pte))
+		if (pte_protnone(pte) && !gup_can_follow_protnone(flags))
 			goto pte_unmap;

 		if (!pte_access_permitted(pte, flags & FOLL_WRITE))
@@ -2736,12 +2732,8 @@ static int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, unsigned lo

 		if (unlikely(pmd_trans_huge(pmd) || pmd_huge(pmd) ||
 			     pmd_devmap(pmd))) {
-			/*
-			 * NUMA hinting faults need to be handled in the GUP
-			 * slowpath for accounting purposes and so that they
-			 * can be serialised against THP migration.
-			 */
-			if (pmd_protnone(pmd))
+			if (pmd_protnone(pmd) &&
+			    !gup_can_follow_protnone(flags))
 				return 0;

 			if (!gup_huge_pmd(pmd, pmdp, addr, next, flags,
-- 
2.37.1