On 27/01/2026 15:03, Ard Biesheuvel wrote:

On Tue, 27 Jan 2026 at 10:45, Ryan Roberts [off-list ref] wrote:

quoted

On 26/01/2026 09:26, Ard Biesheuvel wrote:

quoted

From: Ard Biesheuvel <ardb@kernel.org>

Currently, pgattr_change_is_safe() is overly pedantic when it comes to
descriptors with the contiguous hint attribute set, as it rejects
assignments even if the old and the new value are the same.

So relax the check to allow that.

But why do we require the relaxation? Why are we re-writing a PTE in the first
place? Either the caller already knows it's the same in which case it can be
avoided, or it doesn't know in which case it is accidentally the same and couple
probably just as easily been accidentally different? So it's better to warn
regardless I would think?

Based on rule RJQQTC in your reply to another patch in this series, my
conclusion here is that we can drop this check entirely.

Hmm, I don't think that would be quite right; The rule permits _some_ bits of
the PTE to change in a live mapping as long as the CONT bit remains unchanged.
If you change the CONT bit on a live mapping, you could end up with overlapping
TLB entries which would not go well on a system without bbml2.