linux-kernel - Re: [PATCH v3 2/5] mm/mseal: update madvise() logic

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Message-ID: <649565be-eab2-4783-9595-b4263ac50310@redhat.com>
Date: Fri, 25 Jul 2025 12:10:07 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 Pedro Falcato <pfalcato@...e.de>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Jeff Xu <jeffxu@...omium.org>
Subject: Re: [PATCH v3 2/5] mm/mseal: update madvise() logic

>>> So - we explicitly disallow FOLL_FORCE write override for CoW file-backed
>>> mappings.
>>>
>>> Obviously if FOLL_FORCE is not set, then we're ALSO not allowed to get past a
>>> FOLL_WRITE and !VM_WRITE situation.
>>>
>>>>
>>>>>
>>>>> Hmm maybe I'll soften on this anon_vma idea then. Maybe it is a 'cheap fix' to
>>>>> rule out the _usual_ cases.
>>>>
>>>> Yeah, something to evaluate.
>>>
>>> I'm thinking more and more we're probably actually safe with !vma->anon_vma ||
>>> !(vma->vm_flags & VM_MAYWRITE).
>>
>> I think there are possible races, the question is how much you care about
>> them.
> 
> Yes I was just wrong. Please just disregard.
> 
> I mean racing with MADV_POPULATE_WRITE seems a niche thing to worry about, and
> so what if you did, it's writing a... copy of the underlying file-backed folios
> no?

MADV_POPULATE_WRITE does not apply. It's only racing with FOLL_FORCE, 
like debugger access.

Again, a race one probably shouldn't worry about in the context of mseal.

-- 
Cheers,

David / dhildenb