linux-kernel - Re: [PATCH 2/5] mm/mseal: move madvise() logic to mm/madvise.c

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Message-ID: <184d7f4b-6ffd-4047-b245-aec32ebda9d6@redhat.com>
Date: Mon, 14 Jul 2025 17:41:45 +0200
From: David Hildenbrand <david@...hat.com>
To: Pedro Falcato <pfalcato@...e.de>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Jeff Xu <jeffxu@...omium.org>
Subject: Re: [PATCH 2/5] mm/mseal: move madvise() logic to mm/madvise.c

On 14.07.25 17:31, Pedro Falcato wrote:
> On Mon, Jul 14, 2025 at 05:03:03PM +0200, David Hildenbrand wrote:
>> [...]
>>
>> But now I wonder, why is it okay to discard anon pages in a MAP_PRIVATE file
>> mapping?
> 
> IIRC this was originally suggested by Linus, on one of the versions introducing
> mseal. But the gist is that discarding pages is okay if you could already zero
> them manually, using e.g memset. Hence the writeability checks.

What you can do is

a) mmap(PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, ...)

b) modify content (write, whatever)

c) mprotect(PROT_READ)

d) mseal()

But then still do

madvise(MADV_DONTNEED)

to discard.

There is no writability anymore.

(Just a note that, with hugetlb, it is fairly common to 
mmap(MAP_PRIVATE) empty files and only work with anonymous pages.)

-- 
Cheers,

David / dhildenb