Re: [PATCH v2 2/5] mm: add PTE_MARKER_GUARD PTE marker

[David Hildenbrand <david@redhat.com>]

On 21.10.24 18:23, Lorenzo Stoakes wrote:
> On Mon, Oct 21, 2024 at 06:00:20PM +0200, David Hildenbrand wrote:
> [snip]
>>>
>>> To summarise for on-list:
>>>
>>> * MADV_FREE, while ostensibly being a 'lazy free' mechanism, has the
>>>     ability to be 'cancelled' if you write to the memory. Also, after the
>>>     freeing is complete, you can write to the memory to reuse it, the mapping
>>>     is still there.
>>>
>>> * For hardware poison markers it makes sense to drop them as you're
>>>     effectively saying 'I am done with this range that is now unbacked and
>>>     expect to get an empty page should I use it now'. UFFD WP I am not sure
>>>     about but presumably also fine.
>>>
>>> * However, guard pages are different - if you 'cancel' and you are left
>>>     with a block of memory allocated to you by a pthread or userland
>>>     allocator implementation, you don't want to then no longer be protected
>>>     from overrunning into other thread memory.
>>
>> Agreed. What happens on MADV_DONTNEED/MADV_FREE on guard pages? Ignored or
>> error? It sounds like a usage "error" to me (in contrast to munmap()).
> 
> It's ignored, no errror. On MADV_DONTNEED we already left the guard pages in
> place, from v3 we will do the same for MADV_FREE.
> 
> I'm not sure I'd say it's an error per se, as somebody might have a use case
> where they want to zap over a range but keep guard pages, perhaps an allocator
> or something?

Hm, not sure I see use for that.

Staring at madvise_walk_vmas(), we return ENOMEM on VMA holes, but would 
process PROT_NONE. So current behavior is at least consistent with 
PROT_NONE handling (where something could be mapped, though).

No strong opinion.

> 
> Also the existing logic is that existing markers (HW poison, uffd-simulated HW
> poison, uffd wp marker) are retained and no error raised on MADV_DONTNEED, and
> no error on MADV_FREE either, so it'd be consistent with existing behaviour.


HW poison / uffd-simulated HW poison are expected to be zapped: it's 
just like a mapped page with HWPOISON. So that is correct.
	
UFFD-WP behavior is ... weird. Would not expect MADV_DONTNEED to zap 
uffd-wp entries.

> 
> Also semantically you are achieving what the calls expect you are freeing the
> ranges since the guard page regions are unbacked so are already freed... so yeah
> I don't think an error really makes sense here.

I you compare it to a VMA hole, it make sense to fail. If we treat it 
like PROT_NONE, it make sense to skip them.

> 
> We might also be limiting use cases by assuming they might _only_ be used for
> allocators and such.

I don't buy that as an argument, sorry :)

"Let's map the kernel writable into all user space because otherwise we 
might be limiting use cases"


:P

-- 
Cheers,

David / dhildenb