[PATCH] arm64/mm: Disable barrier batching in interrupt contexts

[Will Deacon]

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