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From: Mike Rapoport <rppt@kernel.org>
To: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Cc: linux-mm@kvack.org, Albert Ou <aou@eecs.berkeley.edu>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Brendan Jackman <jackmanb@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	David Hildenbrand <david@kernel.org>,
	Huacai Chen <chenhuacai@kernel.org>,
	Ingo Molnar <mingo@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Kevin Brodsky <kevin.brodsky@arm.com>,
	"Liam R. Howlett" <liam@infradead.org>,
	Michal Hocko <mhocko@suse.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <pjw@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Thomas Gleixner <tglx@kernel.org>,
	Vlastimil Babka <vbabka@kernel.org>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
	loongarch@lists.linux.dev, x86@kernel.org,
	Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH RFC 01/18] x86/mm/pat: Don't gate cpa_lock on debug_pagealloc_enabled()
Date: Tue, 21 Jul 2026 21:30:26 +0300	[thread overview]
Message-ID: <al-6woCPBH5d-lwP@kernel.org> (raw)
In-Reply-To: <al-oTOKY7vkdPHr5@lucifer>

On Tue, Jul 21, 2026 at 06:16:33PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Tue, Jul 21, 2026 at 07:23:24PM +0300, Mike Rapoport (Microsoft) wrote:
> > The splitting and merging of kernel page table mappings between small and
> > large is protected by cpa_lock. The merging is relatively new but the
> > splitting is ancient.
> >
> > The splitting has a locking optimization: since DEBUG_PAGEALLOC forces all
> > mappings to 4k, there are no large pages to split. So the code that *might*
> > cause a split can just skip the locking (and a few other things).
> >
> > This is entertaining, but it adds complexity and makes for weird locking
> > rules. Plus it's all for a debugging feature which makes the kernel super
> > slow in the first place. Optimizing something which is already super slow
> > and not used in production is not the best way to spend our complexity
> > budget.
> >
> > Stop gating cpa_lock on debug_pagealloc_enabled() to simplify the code
> > and the locking rules.
> >
> > [ dhansen: flesh out changelog ]
> >
> > Suggested-by: Dave Hansen <dave.hansen@intel.com>
> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> > Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> > Link: https://patch.msgid.link/20260715144519.934289-1-rppt@kernel.org
> > Link: https://lore.kernel.org/all/aab44f08-89f8-47fe-bee4-0ab6b25968c6@intel.com/
> 
> Hmm this patch is already taken separately though? ([0]) (obv. commented there
> already with review feedback).
> 
> Intended to be with this series as some kind of background or? Probably better
> to separate out given it's a live patch

From the cover letter:

  Patch 1 is already applied to the tip tree and I posted patch 2
  separately [4], but I included them both here for the completeness.
 
> Thanks, Lorenzo

-- 
Sincerely yours,
Mike.

WARNING: multiple messages have this Message-ID (diff)
From: Mike Rapoport <rppt@kernel.org>
To: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Cc: linux-mm@kvack.org, Albert Ou <aou@eecs.berkeley.edu>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Brendan Jackman <jackmanb@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	David Hildenbrand <david@kernel.org>,
	Huacai Chen <chenhuacai@kernel.org>,
	Ingo Molnar <mingo@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Kevin Brodsky <kevin.brodsky@arm.com>,
	"Liam R. Howlett" <liam@infradead.org>,
	Michal Hocko <mhocko@suse.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <pjw@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Thomas Gleixner <tglx@kernel.org>,
	Vlastimil Babka <vbabka@kernel.org>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
	loongarch@lists.linux.dev, x86@kernel.org,
	Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH RFC 01/18] x86/mm/pat: Don't gate cpa_lock on debug_pagealloc_enabled()
Date: Tue, 21 Jul 2026 21:30:26 +0300	[thread overview]
Message-ID: <al-6woCPBH5d-lwP@kernel.org> (raw)
In-Reply-To: <al-oTOKY7vkdPHr5@lucifer>

On Tue, Jul 21, 2026 at 06:16:33PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Tue, Jul 21, 2026 at 07:23:24PM +0300, Mike Rapoport (Microsoft) wrote:
> > The splitting and merging of kernel page table mappings between small and
> > large is protected by cpa_lock. The merging is relatively new but the
> > splitting is ancient.
> >
> > The splitting has a locking optimization: since DEBUG_PAGEALLOC forces all
> > mappings to 4k, there are no large pages to split. So the code that *might*
> > cause a split can just skip the locking (and a few other things).
> >
> > This is entertaining, but it adds complexity and makes for weird locking
> > rules. Plus it's all for a debugging feature which makes the kernel super
> > slow in the first place. Optimizing something which is already super slow
> > and not used in production is not the best way to spend our complexity
> > budget.
> >
> > Stop gating cpa_lock on debug_pagealloc_enabled() to simplify the code
> > and the locking rules.
> >
> > [ dhansen: flesh out changelog ]
> >
> > Suggested-by: Dave Hansen <dave.hansen@intel.com>
> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> > Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> > Link: https://patch.msgid.link/20260715144519.934289-1-rppt@kernel.org
> > Link: https://lore.kernel.org/all/aab44f08-89f8-47fe-bee4-0ab6b25968c6@intel.com/
> 
> Hmm this patch is already taken separately though? ([0]) (obv. commented there
> already with review feedback).
> 
> Intended to be with this series as some kind of background or? Probably better
> to separate out given it's a live patch

From the cover letter:

  Patch 1 is already applied to the tip tree and I posted patch 2
  separately [4], but I included them both here for the completeness.
 
> Thanks, Lorenzo

-- 
Sincerely yours,
Mike.

_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv

  reply	other threads:[~2026-07-21 18:30 UTC|newest]

Thread overview: 56+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-07-21 16:23 [PATCH RFC 00/18] arch, mm: create a generic set_memory/change_page_attr core Mike Rapoport (Microsoft)
2026-07-21 16:23 ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 01/18] x86/mm/pat: Don't gate cpa_lock on debug_pagealloc_enabled() Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 17:16   ` Lorenzo Stoakes (ARM)
2026-07-21 17:16     ` Lorenzo Stoakes (ARM)
2026-07-21 18:30     ` Mike Rapoport [this message]
2026-07-21 18:30       ` Mike Rapoport
2026-07-22  8:55       ` Lorenzo Stoakes (ARM)
2026-07-22  8:55         ` Lorenzo Stoakes (ARM)
2026-07-21 16:23 ` [PATCH RFC 02/18] x86/mm/pat: fix effective RW computation in lookup_address_in_pgd_attr() Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 03/18] x86/mm/pat: replace x86-speficic pg_level with generic pgtable_level Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 04/18] x86/mm/pat: use page table accessors for effective RW/NX bits Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 05/18] x86/mm/pat: use a generic helper for the large page size Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 06/18] x86/mm/pat: introduce an inline helper to check if alias needs update Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 07/18] x86/mm/pat: introduce cpa_handle_large_page() Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 08/18] x86/mm/pat: use cpa_split_data structure to pass split parameters Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 09/18] x86/mm/pat: use a single locked lookup for PTE verification Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 10/18] x86/mm/pat: move the large page size calculation to should_split_large_page Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 11/18] x86/mm/pat, mm: set_memory: pull change_page_attr() core to mm/ Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 12/18] x86/pat/mm, mm/set_memory: abstract the CPA page table lock Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 13/18] mm/set_memory: read page table entries with pXdp_get() Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 14/18] x86/mm/pat, mm: move cpa-test to mm Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 15/18] riscv: convert set_memory to the generic CPA implementation Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 16/18] riscv: make set_direct_map_*_noflush actually noflush Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 17/18] loongarch: convert set_memory to the generic CPA implementation Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 16:23 ` [PATCH RFC 18/18] loongarch: make set_direct_map_*_noflush actually noflush Mike Rapoport (Microsoft)
2026-07-21 16:23   ` Mike Rapoport (Microsoft)
2026-07-21 17:19 ` [PATCH RFC 00/18] arch, mm: create a generic set_memory/change_page_attr core Borislav Petkov
2026-07-21 17:19   ` Borislav Petkov
2026-07-21 18:28   ` Mike Rapoport
2026-07-21 18:28     ` Mike Rapoport
2026-07-21 19:54     ` Borislav Petkov
2026-07-21 19:54       ` Borislav Petkov
2026-07-22  6:50       ` Mike Rapoport
2026-07-22  6:50         ` Mike Rapoport
2026-07-22 13:20 ` [syzbot ci] " syzbot ci
2026-07-22 13:20   ` syzbot ci
2026-07-22 15:17 ` [PATCH RFC 00/18] " Adrian Barnaś
2026-07-22 15:17   ` Adrian Barnaś

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