Re: [PATCH] arm64: hugetlb: fix BBM for mprotect() on contiguous PTEs

[Karl Mehltretter <kmehltretter@gmail.com>]

On Wed, Sep 02, 2026 at 10:08:58AM +0100, Anshuman Khandual wrote:
> No user-visiable failure was reported even without this change ?
> Missing TLBI as seen in QEMU is the only clue here ?A

This came out of an experiment to add warnings for architectural
details that QEMU does not normally model. The checker tracks guest
page-table writes and TLBIs using a shadow TLB.

It reported 3072 contiguous PTE transitions without an intervening
TLBI on the unpatched kernel, none with this change, and none for the
already-flushed write-protect control. Together with the code history,
that was the basis for the patch.

As noted in the testing section, no user-visible failure was
reproduced. 

I also tested an unpatched kernel on a Raspberry Pi 400.

The Cortex-A72's L1 data-TLB refill ratio was approximately 1:16 for
the 64K hugetlb mapping versus a forced-4K control, indicating that it
amalgamated the 16 contiguous PTEs. A contended test still completed
85,396 full-range permission changes and about 2.8 billion reads
without a fault or data mismatch.

> But you have not seen any real TLB conflicts even on custom QEMU ?

No. QEMU does not model the stage-1 Contiguous hint as a combined TLB
entry. The added checker only logged the page-table/TLBI sequence, it
did not change translation behaviour or inject TLB conflicts.

Thanks,
Karl