linux-kernel - Re: [PATCH v1 0/4] mm/hugetlb: fixes for PMD table sharing (incl. using mmu

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Message-ID: <6640379f1ef7e9305b77d1e322b04d9534a3fff5.camel@redhat.com>
Date: Sat, 06 Dec 2025 14:53:02 -0500
From: Laurence Oberman <loberman@...hat.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>, 
	linux-kernel@...r.kernel.org
Cc: 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>, Prakash Sangappa
 <prakash.sangappa@...cle.com>, Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH v1 0/4] mm/hugetlb: fixes for PMD table sharing (incl.
 using mmu_gather)

On Fri, 2025-12-05 at 22:35 +0100, David Hildenbrand (Red Hat) wrote:
> One functional fix, one performance regression fix, and two related
> comment fixes.
> 
> I cleaned up my prototype I recently shared [1] for the performance
> fix,
> deferring most of the cleanups I had in the prototype to a later
> point.
> While doing that I identified the other things.
> 
> The goal of this patch set is to be backported to stable trees
> "fairly"
> easily. At least patch #1 and #4.
> 
> Patch #1 fixes hugetlb_pmd_shared() not detecting any sharing
> Patch #2 + #3 are simple comment fixes that patch #4 interacts with.
> Patch #4 is a fix for the reported performance regression due to
> excessive
> IPI broadcasts during fork()+exit().
> 
> The last patch is all about TLB flushes, IPIs and mmu_gather.
> Read: complicated
> 
> I added as much comments + description that I possibly could, and I
> am
> hoping for review from Jann.
> 
> There are plenty of cleanups in the future to be had + one reasonable
> optimization on x86. But that's all out of scope for this series.
> 
> Compile tested on plenty of architectures.
> 
> Runtime tested, with a focus on fixing the performance regression
> using
> the original reproducer [2] on x86.
> 
> I'm still busy with more testing (making sure that my TLB flushing
> changes
> are good), but sending this out already so people can test and review
> while I am soon heading for LPC.
> 
> [1]
> https://lore.kernel.org/all/8cab934d-4a56-44aa-b641-bfd7e23bd673@kernel.org/
> [2]
> https://lore.kernel.org/all/8cab934d-4a56-44aa-b641-bfd7e23bd673@kernel.org/
> 
> Cc: Will Deacon <will@...nel.org>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Nick Piggin <npiggin@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Muchun Song <muchun.song@...ux.dev>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Jann Horn <jannh@...gle.com>
> Cc: Pedro Falcato <pfalcato@...e.de>
> Cc: Rik van Riel <riel@...riel.com>
> Cc: Harry Yoo <harry.yoo@...cle.com>
> Cc: Uschakow, Stanislav" <suschako@...zon.de>
> Cc: Laurence Oberman <loberman@...hat.com>
> Cc: Prakash Sangappa <prakash.sangappa@...cle.com>
> Cc: Nadav Amit <nadav.amit@...il.com>
> 
> David Hildenbrand (Red Hat) (4):
>   mm/hugetlb: fix hugetlb_pmd_shared()
>   mm/hugetlb: fix two comments related to huge_pmd_unshare()
>   mm/rmap: fix two comments related to huge_pmd_unshare()
>   mm/hugetlb: fix excessive IPI broadcasts when unsharing PMD tables
>     using mmu_gather
> 
>  include/asm-generic/tlb.h |  69 +++++++++++++++++++-
>  include/linux/hugetlb.h   |  21 ++++---
>  mm/hugetlb.c              | 129 ++++++++++++++++++++----------------
> --
>  mm/mmu_gather.c           |   6 ++
>  mm/mprotect.c             |   2 +-
>  mm/rmap.c                 |  45 +++++++------
>  6 files changed, 178 insertions(+), 94 deletions(-)
> 

For the Series passed generic testing with a focus on the CVE
regression and looks good.

Tested-by: Laurence Oberman <loberman@...hat.com>