linux-kernel - Re: [PATCH] arm64/mm: Disable barrier batching in interrupt contexts

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Message-ID: <20250513204603.GA9866@willie-the-truck>
Date: Tue, 13 May 2025 21:46:04 +0100
From: Will Deacon <will@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	David Hildenbrand <david@...hat.com>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Mark Rutland <mark.rutland@....com>,
	Anshuman Khandual <anshuman.khandual@....com>,
	Alexandre Ghiti <alexghiti@...osinc.com>,
	Kevin Brodsky <kevin.brodsky@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	syzbot+5c0d9392e042f41d45c5@...kaller.appspotmail.com
Subject: Re: [PATCH] arm64/mm: Disable barrier batching in interrupt contexts

On Mon, May 12, 2025 at 11:22:40AM +0100, Ryan Roberts wrote:
> Commit 5fdd05efa1cd ("arm64/mm: Batch barriers when updating kernel
> mappings") enabled arm64 kernels to track "lazy mmu mode" using TIF
> flags in order to defer barriers until exiting the mode. At the same
> time, it added warnings to check that pte manipulations were never
> performed in interrupt context, because the tracking implementation
> could not deal with nesting.
> 
> But it turns out that some debug features (e.g. KFENCE, DEBUG_PAGEALLOC)
> do manipulate ptes in softirq context, which triggered the warnings.

Hmm. Do we also have to worry about the case where a softirq is triggered
off the back of a hardirq *and* that hardirq is taken while we're in the
middle of e.g. queue_pte_barriers()? In that case, I think we can end
up in strange situations, such as having LAZY_MMU_PENDING set when
LAZY_MMU is clear, although it looks like things still work even in that
case.

Will