Re: [tip: x86/urgent] x86/mm/pat: Fix effective RW computation in lookup_address_in_pgd_attr()

[Dave Hansen]

On 9/6/26 11:12, Nathan Chancellor wrote:
> On Sun, Sep 06, 2026 at 07:50:26AM -0700, Dave Hansen wrote:
>> On 9/4/26 21:42, Nathan Chancellor wrote:
>>> I just bisected the following warning on a couple of my test machines to
>>> commit 453e78594434 ("x86/mm/pat: Fix effective RW computation in
>>> lookup_address_in_pgd_attr()") in next-20260904.
>> ...
>>> If there is any information I can provide to help debug this, I am happy
>>> to provide it.
>>
>> Well, the thing you bisected to is the patch that fixes the warning. The
> 
> Hmmm, I don't see a warning prior to this patch though. Do you mean that
> this patch is just revealing the bad behavior that you mention below?

Yeah, "fixes" was a bad word to use.

The W^X detection has been broken for a while. The patch you bisected to
fixes the W^X detection. It doesn't actually cause the issue.

>> actual issue comes from:
>>
>>         if (pages == &its_pages)
>>                 set_memory_x((unsigned long)page, 1);
>>
>> which its_alloc() does on an execmem allocation. Any chance you could
>> get the output of:
>>
>> 	/sys/kernel/debug/page_tables/current_kernel
>>
>> for the page that page that it's complaining about? I'm curious if this
>> is temporary or permanent.
> 
> Sure. For
> 
>    CPA detected W^X violation: 8000000000000123 -> 0000000000000123 range: 0xffffffffc0200000 - 0xffffffffc0200fff PFN 100e00
> 
> I think it is this line if I understand correctly?
> 
>   0xffffffffc0200000-0xffffffffc0400000           2M     ro         PSE     GLB x  pmd

Yeah, that does seems to show it getting fixed up. Thanks! That means
that the warning (at least in early boot) is essentially a false
positive. The problem is that there's module-load-time patching too and
that is much later and is arguably an actual problem.

Oh, and thinking about it a bit more... This warning came from
alternatives patching which is too early for normal text_poke(), thus
the existence of text_poke_early().

Anyway, I'm curious what Peter wants to do with this.