On Mon, 27 Jul 2020 16:09:47 +1000, Nicholas Piggin wrote:

Commit 2f92447f9f96 ("powerpc/book3s64/hash: Use the pte_t address from the
caller") removed the local_irq_disable from hash_preload, but it was
required for more than just the page table walk: the hash pte busy bit is
effectively a lock which may be taken in interrupt context, and the local
update flag test must not be preempted before it's used.

This solves apparent lockups with perf interrupting __hash_page_64K. If
get_perf_callchain then also takes a hash fault on the same page while it
is already locked, it will loop forever taking hash faults, which looks like
this:

[...]

Applied to powerpc/fixes.

[1/1] powerpc/64s/hash: Fix hash_preload running with interrupts enabled
      https://git.kernel.org/powerpc/c/909adfc66b9a1db21b5e8733e9ebfa6cd5135d74

cheers