linux-kernel - [PATCH v5 01/25] mm: Clarify the spec for set

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Date: Fri,  2 Feb 2024 08:07:32 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Ard Biesheuvel <ardb@...nel.org>,
	Marc Zyngier <maz@...nel.org>,
	James Morse <james.morse@....com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <willy@...radead.org>,
	Mark Rutland <mark.rutland@....com>,
	David Hildenbrand <david@...hat.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	John Hubbard <jhubbard@...dia.com>,
	Zi Yan <ziy@...dia.com>,
	Barry Song <21cnbao@...il.com>,
	Alistair Popple <apopple@...dia.com>,
	Yang Shi <shy828301@...il.com>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	"Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
	"Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Cc: Ryan Roberts <ryan.roberts@....com>,
	linux-arm-kernel@...ts.infradead.org,
	x86@...nel.org,
	linuxppc-dev@...ts.ozlabs.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v5 01/25] mm: Clarify the spec for set_ptes()

set_ptes() spec implies that it can only be used to set a present pte
because it interprets the PFN field to increment it. However,
set_pte_at() has been implemented on top of set_ptes() since set_ptes()
was introduced, and set_pte_at() allows setting a pte to a not-present
state. So clarify the spec to state that when nr==1, new state of pte
may be present or not present. When nr>1, new state of all ptes must be
present.

While we are at it, tighten the spec to set requirements around the
initial state of ptes; when nr==1 it may be either present or
not-present. But when nr>1 all ptes must initially be not-present. All
set_ptes() callsites already conform to this requirement. Stating it
explicitly is useful because it allows for a simplification to the
upcoming arm64 contpte implementation.

Signed-off-by: Ryan Roberts <ryan.roberts@....com>
---
 include/linux/pgtable.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index f0feae7f89fb..5e7eaf8f2b97 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -229,6 +229,10 @@ static inline pte_t pte_next_pfn(pte_t pte)
  * @pte: Page table entry for the first page.
  * @nr: Number of pages to map.
  *
+ * When nr==1, initial state of pte may be present or not present, and new state
+ * may be present or not present. When nr>1, initial state of all ptes must be
+ * not present, and new state must be present.
+ *
  * May be overridden by the architecture, or the architecture can define
  * set_pte() and PFN_PTE_SHIFT.
  *
-- 
2.25.1