linux-kernel - Re: [PATCH 0/4] mm: permit guard regions for file-backed/shmem mappings

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Message-ID: <4f5a9c19-9bdd-47eb-bb14-205e3dcced90@redhat.com>
Date: Thu, 20 Feb 2025 11:03:02 +0100
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Kalesh Singh <kaleshsingh@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Suren Baghdasaryan <surenb@...gle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Matthew Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>,
 "Paul E . McKenney" <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org,
 linux-api@...r.kernel.org, John Hubbard <jhubbard@...dia.com>,
 Juan Yescas <jyescas@...gle.com>
Subject: Re: [PATCH 0/4] mm: permit guard regions for file-backed/shmem
 mappings

>> Your conclusion is 'did not participate with upstream'; I don't agree with
>> that. But maybe you and Kalesh have a history on that that let's you react
>> on his questions IMHO more emotionally than it should have been.
> 
> This is wholly unfair, I have been very reasonable in response to this
> thread. I have offered to find solutions, I have tried to understand the
> problem in spite of having gone to great lengths to try to discuss the
> limitations of the proposed approach in every venue I possibly could.
> 
> I go out of my way to deal professionally and objectively with what is
> presented. Nothing here is emotional. So I'd ask that you please abstain
> from making commentary like this which has no basis.

I appreciate everything you write below. But this request is just 
impossible. I will keep raising my opinion and misunderstandings will 
happen.

Note that the whole "Honestly David you and the naming. .." thing could 
have been written as "I don't think it's a naming problem."

>>
>>
>>>
>>>>
>>>> [...]
>>>>
>>>>>
>>>>> As I said to you earlier, the _best_ we could do in smaps would be to add a
>>>>> flag like 'Grd' or something to indicate some part of the VMA is
>>>>> guarded. But I won't do that unless somebody has an -actual use case- for
>>>>> it.
>>>>
>>>> Right, and that would limit where you have to manually scan. Something
>>>> similar is being done with uffd-wp markers IIRC.
>>>
>>> Yeah that's a good point, but honestly if you're reading smaps that reads
>>> the page tables, then reading /proc/$pid/pagemaps and reading page tables
>>> TWICE that seems inefficient vs. just reading /proc/$pid/maps, then reading
>>> /proc/$pid/pagemaps and reading page tables once.
>>
>> Right; I recently wished that we would have an interface to obtain more VMA
>> flags without having to go through smaps
> 
> Well maybe that lends itself to the idea of adding a whole new interface in
> general...

An extended "maps" interface might be reasonable, that allows for 
exposing more things without walking the page tables. (e.g., flags)

Maybe one could have an indicator that says "ever had guard regions in 
this mapping" without actually walking the page tables.

-- 
Cheers,

David / dhildenb