linux-kernel - [PATCH 06/19] mm/pagewalk: Check pfnmap early for folio_walk

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Message-ID: <20240809160909.1023470-7-peterx@redhat.com>
Date: Fri,  9 Aug 2024 12:08:56 -0400
From: Peter Xu <peterx@...hat.com>
To: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Cc: Sean Christopherson <seanjc@...gle.com>,
	Oscar Salvador <osalvador@...e.de>,
	Jason Gunthorpe <jgg@...dia.com>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	linux-arm-kernel@...ts.infradead.org,
	x86@...nel.org,
	peterx@...hat.com,
	Will Deacon <will@...nel.org>,
	Gavin Shan <gshan@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Zi Yan <ziy@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Alistair Popple <apopple@...dia.com>,
	Borislav Petkov <bp@...en8.de>,
	David Hildenbrand <david@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	kvm@...r.kernel.org,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Yan Zhao <yan.y.zhao@...el.com>
Subject: [PATCH 06/19] mm/pagewalk: Check pfnmap early for folio_walk_start()

Pfnmaps can always be identified with special bits in the ptes/pmds/puds.
However that's unnecessary if the vma is stable, and when it's mapped under
VM_PFNMAP | VM_IO.

Instead of adding similar checks in all the levels for huge pfnmaps, let
folio_walk_start() fail even earlier for these mappings.  It's also
something gup-slow already does, so make them match.

Cc: David Hildenbrand <david@...hat.com>
Signed-off-by: Peter Xu <peterx@...hat.com>
---
 mm/pagewalk.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index cd79fb3b89e5..fd3965efe773 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -727,6 +727,11 @@ struct folio *folio_walk_start(struct folio_walk *fw,
 	p4d_t *p4dp;
 
 	mmap_assert_locked(vma->vm_mm);
+
+	/* It has no folio backing the mappings at all.. */
+	if (vma->vm_flags & (VM_IO | VM_PFNMAP))
+		return NULL;
+
 	vma_pgtable_walk_begin(vma);
 
 	if (WARN_ON_ONCE(addr < vma->vm_start || addr >= vma->vm_end))
-- 
2.45.0