Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key faults

From: Dave Hansen
Date: Thu Sep 24 2015 - 13:41:18 EST

On 09/24/2015 02:30 AM, Ingo Molnar wrote:
>> To answer your question in the comment: it looks useful to have some sort of
>> 'extended page fault error code' information here, which shows why the page fault
>> happened. With the regular error_code it's easy - with protection keys there's 16
>> separate keys possible and user-space might not know the actual key value in the
>> pte.
> Btw., alternatively we could also say that user-space should know what protection
> key it used when it created the mapping - there's no need to recover it for every
> page fault.

That's true. We don't, for instance, tell userspace whether it was a
write that caused a fault.

But, other than smaps we don't have *any* way to tell userspace what
protection key a page has. I think some mechanism is going to be
required for this to be reasonably debuggable.

> OTOH, as long as we don't do a separate find_vma(), it looks cheap enough to look
> up the pkey value of that address and give it to user-space in the signal frame.

I still think that find_vma() in this case is pretty darn cheap,
definitely if you compare it to the cost of the entire fault path.

> Btw., how does pkey support interact with hugepages?

Surprisingly little. I've made sure that everything works with huge
pages and that the (huge) PTEs and VMAs get set up correctly, but I'm
not sure I had to touch the huge page code at all. I have test code to
ensure that it works the same as with small pages, but everything worked
pretty naturally.
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