linux-kernel - [PATCH 5.15 05/37] powerpc/64s/radix: dont need to broadcast IPI for radix pmd collapse flush

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Date:   Mon, 10 Oct 2022 09:05:24 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Hugh Dickins <hughd@...gle.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 5.15 05/37] powerpc/64s/radix: dont need to broadcast IPI for radix pmd collapse flush

From: Yang Shi <shy828301@...il.com>

commit bedf03416913d88c796288f9dca109a53608c745 upstream.

The IPI broadcast is used to serialize against fast-GUP, but fast-GUP will
move to use RCU instead of disabling local interrupts in fast-GUP.  Using
an IPI is the old-styled way of serializing against fast-GUP although it
still works as expected now.

And fast-GUP now fixed the potential race with THP collapse by checking
whether PMD is changed or not.  So IPI broadcast in radix pmd collapse
flush is not necessary anymore.  But it is still needed for hash TLB.

Link: https://lkml.kernel.org/r/20220907180144.555485-2-shy828301@gmail.com
Suggested-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
Signed-off-by: Yang Shi <shy828301@...il.com>
Acked-by: David Hildenbrand <david@...hat.com>
Acked-by: Peter Xu <peterx@...hat.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Jason Gunthorpe <jgg@...dia.com>
Cc: John Hubbard <jhubbard@...dia.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/powerpc/mm/book3s64/radix_pgtable.c |    9 ---------
 1 file changed, 9 deletions(-)

--- a/arch/powerpc/mm/book3s64/radix_pgtable.c
+++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
@@ -954,15 +954,6 @@ pmd_t radix__pmdp_collapse_flush(struct
 	pmd = *pmdp;
 	pmd_clear(pmdp);
 
-	/*
-	 * pmdp collapse_flush need to ensure that there are no parallel gup
-	 * walk after this call. This is needed so that we can have stable
-	 * page ref count when collapsing a page. We don't allow a collapse page
-	 * if we have gup taken on the page. We can ensure that by sending IPI
-	 * because gup walk happens with IRQ disabled.
-	 */
-	serialize_against_pte_lookup(vma->vm_mm);
-
 	radix__flush_tlb_collapsed_pmd(vma->vm_mm, address);
 
 	return pmd;