linux-kernel - Re: [PATCH v3 0/4] move all VMA allocation, freeing and duplication logic to mm

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Message-ID: <e5564971-b632-4619-829e-342cdad02e25@lucifer.local>
Date: Tue, 29 Apr 2025 11:23:25 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        David Hildenbrand <david@...hat.com>, Kees Cook <kees@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        Suren Baghdasaryan <surenb@...gle.com>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/4] move all VMA allocation, freeing and duplication
 logic to mm

On Tue, Apr 29, 2025 at 09:28:05AM +0200, Vlastimil Babka wrote:
> On 4/28/25 17:28, Lorenzo Stoakes wrote:
> > Currently VMA allocation, freeing and duplication exist in kernel/fork.c,
> > which is a violation of separation of concerns, and leaves these functions
> > exposed to the rest of the kernel when they are in fact internal
> > implementation details.
> >
> > Resolve this by moving this logic to mm, and making it internal to vma.c,
> > vma.h.
> >
> > This also allows us, in future, to provide userland testing around this
> > functionality.
> >
> > We additionally abstract dup_mmap() to mm, being careful to ensure
> > kernel/fork.c acceses this via the mm internal header so it is not exposed
> > elsewhere in the kernel.
> >
> > As part of this change, also abstract initial stack allocation performed in
> > __bprm_mm_init() out of fs code into mm via the create_init_stack_vma(), as
> > this code uses vm_area_alloc() and vm_area_free().
> >
> > In order to do so sensibly, we introduce a new mm/vma_exec.c file, which
> > contains the code that is shared by mm and exec. This file is added to both
> > memory mapping and exec sections in MAINTAINERS so both sets of maintainers
> > can maintain oversight.
>
> Note that kernel/fork.c itself belongs to no section. Maybe we could put it
> somewhere too, maybe also multiple subsystems? I'm thinking something
> between MM, SCHEDULER, EXEC, perhaps PIDFD?

Thanks, indeed I was wondering about where this should be, and the fact we can
put stuff in multiple places is actually pretty powerful!

This is on my todo, will take a look at this.