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

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Date: Thu, 25 Jan 2024 18:06:20 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Nanyong Sun <sunnanyong@...wei.com>
Cc: will@...nel.org, mike.kravetz@...cle.com, muchun.song@...ux.dev,
	akpm@...ux-foundation.org, anshuman.khandual@....com,
	willy@...radead.org, wangkefeng.wang@...wei.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH v3 0/3] A Solution to Re-enable hugetlb vmemmap optimize

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