linux-kernel - Re: [PATCH v1 4/4] mm/hugetlb: fix excessive IPI broadcasts when unsharing PMD tables using mmu

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Message-ID: <3a408dd9-a38c-4202-b79f-91bff843ee82@kernel.org>
Date: Sun, 7 Dec 2025 13:39:10 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Nadav Amit <nadav.amit@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
 linux-mm@...ck.org, Will Deacon <will@...nel.org>,
 "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>, Nick Piggin <npiggin@...il.com>,
 Peter Zijlstra <peterz@...radead.org>, Arnd Bergmann <arnd@...db.de>,
 Muchun Song <muchun.song@...ux.dev>, Oscar Salvador <osalvador@...e.de>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 Pedro Falcato <pfalcato@...e.de>, Rik van Riel <riel@...riel.com>,
 Harry Yoo <harry.yoo@...cle.com>, Laurence Oberman <loberman@...hat.com>,
 Prakash Sangappa <prakash.sangappa@...cle.com>, stable@...r.kernel.org
Subject: Re: [PATCH v1 4/4] mm/hugetlb: fix excessive IPI broadcasts when
 unsharing PMD tables using mmu_gather

On 12/7/25 13:24, Nadav Amit wrote:
> 
> 
>> On 7 Dec 2025, at 14:15, Nadav Amit <nadav.amit@...il.com> wrote:
>>
>>
>>> On 5 Dec 2025, at 23:35, David Hildenbrand (Red Hat) <david@...nel.org> wrote:
>>>
>>> @@ -400,6 +411,7 @@ static inline void __tlb_reset_range(struct mmu_gather *tlb)
>>>
>>
>> @@ -400,6 +411,7 @@ static inline void __tlb_reset_range(struct mmu_gather *tlb)
>> 	tlb->cleared_pmds = 0;
>> 	tlb->cleared_puds = 0;
>> 	tlb->cleared_p4ds = 0;
>> +	tlb->unshared_tables = 0;
>> 	/*
>> 	 * Do not reset mmu_gather::vma_* fields here, we do not
>> 	 * call into tlb_start_vma() again to set them if there is an
>>
>> I understand you don’t want to initialize fully_unshared_tables here, but
>> tlb_gather_mmu() needs to happen somewhere. So you probably want it to
>> take place in tlb_gather_mmu(), no?
> 
> To clarify my messed up response: the code needs to initialize fully_unshared_tables
> somewhere during tlb_gather_mmu() invocation.

Good point, __tlb_gather_mmu() needs to initialize it explicitly!

Thanks a lot for the review, appreciated!

-- 
Cheers

David