linux-kernel - [v2 PATCH 2/2] powerpc/64s/radix: don't need to broadcast IPI for radix pmd collapse flush

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Date:   Wed,  7 Sep 2022 11:01:44 -0700
From:   Yang Shi <shy828301@...il.com>
To:     david@...hat.com, peterx@...hat.com,
        kirill.shutemov@...ux.intel.com, jhubbard@...dia.com,
        jgg@...dia.com, hughd@...gle.com, akpm@...ux-foundation.org,
        aneesh.kumar@...ux.ibm.com
Cc:     shy828301@...il.com, linux-mm@...ck.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: [v2 PATCH 2/2] powerpc/64s/radix: don't need to broadcast IPI for radix pmd collapse flush

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.

Suggested-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
Signed-off-by: Yang Shi <shy828301@...il.com>
---
 arch/powerpc/mm/book3s64/radix_pgtable.c | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/arch/powerpc/mm/book3s64/radix_pgtable.c b/arch/powerpc/mm/book3s64/radix_pgtable.c
index 698274109c91..e712f80fe189 100644
--- a/arch/powerpc/mm/book3s64/radix_pgtable.c
+++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
@@ -937,15 +937,6 @@ pmd_t radix__pmdp_collapse_flush(struct vm_area_struct *vma, unsigned long addre
 	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;
-- 
2.26.3