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

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Message-ID: <aCHUwZzeI6sEbg2T@arm.com>
Date: Mon, 12 May 2025 12:00:17 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Will Deacon <will@...nel.org>,
	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.
> 
> So let's take the simplest and safest route and disable the batching
> optimization in interrupt contexts. This makes these users no worse off
> than prior to the optimization. Additionally the known offenders are
> debug features that only manipulate a single PTE, so there is no
> performance gain anyway.
> 
> There may be some obscure case of encrypted/decrypted DMA with the
> dma_free_coherent called from an interrupt context, but again, this is
> no worse off than prior to the commit.
> 
> Some options for supporting nesting were considered, but there is a
> difficult to solve problem if any code manipulates ptes within interrupt
> context but *outside of* a lazy mmu region. If this case exists, the
> code would expect the updates to be immediate, but because the task
> context may have already been in lazy mmu mode, the updates would be
> deferred, which could cause incorrect behaviour. This problem is avoided
> by always ensuring updates within interrupt context are immediate.
> 
> Fixes: 5fdd05efa1cd ("arm64/mm: Batch barriers when updating kernel mappings")
> Reported-by: syzbot+5c0d9392e042f41d45c5@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/linux-arm-kernel/681f2a09.050a0220.f2294.0006.GAE@google.com/
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>

As per the request in the original report, please also add:

Reported-by: syzbot+5c0d9392e042f41d45c5@...kaller.appspotmail.com

I'll give it a try as well with my configurations and let you know if
there are any problems. In the meantime:

Reviewed-by: Catalin Marinas <catalin.marinas@....com>