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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Apr 25, 2025 at 11:10:00AM +0100, Lorenzo Stoakes wrote:
> On Thu, Apr 24, 2025 at 06:37:39PM -0700, Suren Baghdasaryan wrote:
> > On Thu, Apr 24, 2025 at 6:22 PM Suren Baghdasaryan <surenb@google.com> wrote:
> > >
> > > On Thu, Apr 24, 2025 at 2:22 PM David Hildenbrand <david@redhat.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
> > >
> > > > > 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.
> > > 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
> > >
> > > /* Functions identical for MMU/NOMMU */
> > > struct vm_area_struct *vm_area_alloc(struct mm_struct *mm) {...}
> > > void __init vma_state_init(void) {...}
> > >
> > > #ifdef CONFIG_MMU
> > > static void vm_area_init_from(const struct vm_area_struct *src,
> > >                              struct vm_area_struct *dest) {...}
> > > struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) {...}
> > > void vm_area_free(struct vm_area_struct *vma) {...}
> > > #else /* CONFIG_MMU */
> > > static void vm_area_init_from(const struct vm_area_struct *src,
> > >                              struct vm_area_struct *dest) {...}
> > > struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) {...}
> > > void vm_area_free(struct vm_area_struct *vma) {...}
> > > #endif /* CONFIG_MMU */
> >
> > 3/4 and 4/4 look reasonable but they can change substantially
> > depending on your answer to my suggestion above, so I'll wait for your
> > answer before moving forward.
> > Thanks for doing this!
> > Suren.
>
> You're welcome :)
>
> Well I will be fixing the issue David raised of course :) but as stated in
> previous email, I don't feel it makes sense to put nommu stuff in vma.c really.

UPDATE: As per discussions with Liam, will be going ahead with an alternative
but equivalent approach.

Thanks to both of you for your suggestions on this!

Cheers, Lorenzo

>
> >
> > >
> > >
> > >
> > >
> > >
> > > >
> > > > --
> > > > Cheers,
> > > >
> > > > David / dhildenb
> > > >