Re: [PATCH v3 0/3] A Solution to Re-enable hugetlb vmemmap optimize

From: Catalin Marinas <catalin.marinas@arm.com>
To: Nanyong Sun <sunnanyong@huawei.com>
Cc: will@kernel.org, mike.kravetz@oracle.com, muchun.song@linux.dev,
	akpm@linux-foundation.org, anshuman.khandual@arm.com,
	willy@infradead.org, wangkefeng.wang@huawei.com,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v3 0/3] A Solution to Re-enable hugetlb vmemmap optimize
Date: Thu, 25 Jan 2024 18:06:20 +0000	[thread overview]
Message-ID: <ZbKjHHeEdFYY1xR5@arm.com> (raw)
In-Reply-To: <20240113094436.2506396-1-sunnanyong@huawei.com>

On Sat, Jan 13, 2024 at 05:44:33PM +0800, Nanyong Sun wrote:
> HVO was previously disabled on arm64 [1] due to the lack of necessary
> BBM(break-before-make) logic when changing page tables.
> This set of patches fix this by adding necessary BBM sequence when
> changing page table, and supporting vmemmap page fault handling to
> fixup kernel address translation fault if vmemmap is concurrently accessed.

I'm not keen on this approach. I'm not even sure it's safe. In the
second patch, you take the init_mm.page_table_lock on the fault path but
are we sure this is unlocked when the fault was taken? Basically you can
get a fault anywhere something accesses a struct page.

How often is this code path called? I wonder whether a stop_machine()
approach would be simpler.

Andrew, I'd suggest we drop these patches from the mm tree for the time
being. They haven't received much review from the arm64 folk. Thanks.

-- 
Catalin

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