Re: [PATCH v2 5/8] riscv: mm: Take memory hotplug read-lock during kernel page table dump

From: Oscar Salvador <osalvador@suse.de>
To: "Björn Töpel" <bjorn@kernel.org>
Cc: "Albert Ou" <aou@eecs.berkeley.edu>,
	"Alexandre Ghiti" <alexghiti@rivosinc.com>,
	"Chethan Seshadri" <Chethan.Seshadri@catalinasystems.io>,
	"David Hildenbrand" <david@redhat.com>,
	linux-kernel@vger.kernel.org,
	"Andrew Bresticker" <abrestic@rivosinc.com>,
	"Björn Töpel" <bjorn@rivosinc.com>,
	"Santosh Mamila" <santosh.mamila@catalinasystems.io>,
	linux-mm@kvack.org,
	"Sivakumar Munnangi" <siva.munnangi@catalinasystems.io>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	linux-riscv@lists.infradead.org,
	virtualization@lists.linux-foundation.org,
	"Lorenzo Stoakes" <lstoakes@gmail.com>
Subject: Re: [PATCH v2 5/8] riscv: mm: Take memory hotplug read-lock during kernel page table dump
Date: Tue, 14 May 2024 23:03:39 +0200	[thread overview]
Message-ID: <ZkPRq5zca7PGsqwJ@localhost.localdomain> (raw)
In-Reply-To: <20240514140446.538622-6-bjorn@kernel.org>

On Tue, May 14, 2024 at 04:04:43PM +0200, Björn Töpel wrote:
> From: Björn Töpel <bjorn@rivosinc.com>
> 
> During memory hot remove, the ptdump functionality can end up touching
> stale data. Avoid any potential crashes (or worse), by holding the
> memory hotplug read-lock while traversing the page table.
> 
> This change is analogous to arm64's commit bf2b59f60ee1 ("arm64/mm:
> Hold memory hotplug lock while walking for kernel page table dump").
> 
> Signed-off-by: Björn Töpel <bjorn@rivosinc.com>

Reviewed-by: Oscar Salvador <osalvador@suse.de>

funny enough, it seems arm64 and riscv are the only ones holding the
hotplug lock here.
I think we have the same problem on the other arches as well (at least
on x86_64 that I can see).

If we happen to finally need the lock in those, I would rather have a
centric function in the generic mm code with the locking and then
calling an arch specific ptdump_show function, so the lock is not
scattered. But that is another story.

-- 
Oscar Salvador
SUSE Labs

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