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

[Suren Baghdasaryan <surenb@google.com>]

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 */





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