linux-kernel - Re: [PATCH 2/4] mm: perform VMA allocation, freeing, duplication in mm

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Message-ID: <14616df5-319c-4602-b7a4-f74f988b91c0@lucifer.local>
Date: Fri, 25 Apr 2025 11:31:12 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, Kees Cook <kees@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] mm: perform VMA allocation, freeing, duplication in
 mm

On Fri, Apr 25, 2025 at 06:26:00AM -0400, Liam R. Howlett wrote:
> * Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [250425 06:09]:
> > On Thu, Apr 24, 2025 at 06:22:30PM -0700, Suren Baghdasaryan wrote:
> > > On Thu, Apr 24, 2025 at 2:22 PM David Hildenbrand <david@...hat.com> wrote:
> > > >
> > > > On 24.04.25 23:15, Lorenzo Stoakes wrote:
> > > > > Right now these are performed in kernel/fork.c which is odd and a violation
> > > > > of separation of concerns, as well as preventing us from integrating this
> > > > > and related logic into userland VMA testing going forward, and perhaps more
> > > > > importantly - enabling us to, in a subsequent commit, make VMA
> > > > > allocation/freeing a purely internal mm operation.
> > > > >
> > > > > There is a fly in the ointment - nommu - mmap.c is not compiled if
> > > > > CONFIG_MMU is not set, and there is no sensible place to put these outside
> > > > > of that, so we are put in the position of having to duplication some logic
> > >
> > > s/to duplication/to duplicate
> >
> > Ack will fix!
> >
> > >
> > > > > here.
> > > > >
> > > > > This isn't ideal, but since nommu is a niche use-case, already duplicates a
> > > > > great deal of mmu logic by its nature and we can eliminate code that is not
> > > > > applicable to nommu, it seems a worthwhile trade-off.
> > > > >
> > > > > The intent is to move all this logic to vma.c in a subsequent commit,
> > > > > rendering VMA allocation, freeing and duplication mm-internal-only and
> > > > > userland testable.
> > > >
> > > > I'm pretty sure you tried it, but what's the big blocker to have patch
> > > > #3 first, so we can avoid the temporary move of the code to mmap.c ?
> > >
> > > Completely agree with David.
> >
> > Ack! Yes this was a little bit of a silly one :P
> >
> > > I peeked into 4/4 and it seems you want to keep vma.c completely
> > > CONFIG_MMU-centric. I know we treat NOMMU as an unwanted child but
> > > IMHO it would be much cleaner to move these functions into vma.c from
> > > the beginning and have an #ifdef CONFIG_MMU there like this:
> > >
> > > mm/vma.c
> >
> > This isn't really workable, as the _entire file_ basically contains
> > CONFIG_MMU-specific stuff. so it'd be one huge #ifdef CONFIG_MMU block with
> > one small #else block. It'd also be asking for bugs and issues in nommu.
> >
> > I think doing it this way fits the patterns we have established for
> > nommu/mmap separation, and I would say nommu is enough of a niche edge case
> > for us to really not want to have to go to great lengths to find ways of
> > sharing code.
> >
> > I am quite concerned about us having to consider it and deal with issues
> > around it so often, so want to try to avoid that as much as we can,
> > ideally.
>
> I think you're asking for more issues the way you have it now.  It could
> be a very long time until someone sees that nommu isn't working,
> probably an entire stable kernel cycle.  Basically the longest time it
> can go before being deemed unnecessary to fix.
>
> It could also be worse, it could end up like the arch code with bugs
> over a decade old not being noticed because it was forked off into
> another file.
>
> Could we create another file for the small section of common code and
> achieve your goals?

That'd completely defeat the purpose of isolating core functions to vma.c.

Again, I don't believe that bending over backwards to support this niche
use is appropriate.

And if you're making a change to vm_area_alloc(), vm_area_free(),
vm_area_init_from(), vm_area_dup() it'd seem like an oversight not to check
nommu right?

There's already a large amount of duplicated logic there specific to nommu
for which precisely the same could be said, including entirely parallel
brk(), mmap() implementations.

So this isn't a change in how we handle nommu.

>
> Thanks,
> Liam
>
>