linux-kernel - Re: [PATCH v4 10/21] mm/mmap: Support vma == NULL in init_vma

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Message-ID: <f531253a-f2cd-4e1c-999b-5833a6468241@lucifer.local>
Date: Thu, 11 Jul 2024 15:28:07 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Lorenzo Stoakes <lstoakes@...il.com>,
        Matthew Wilcox <willy@...radead.org>, sidhartha.kumar@...cle.com,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Bert Karwatzki <spasswolf@....de>, Jiri Olsa <olsajiri@...il.com>,
        linux-kernel@...r.kernel.org, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v4 10/21] mm/mmap: Support vma == NULL in
 init_vma_munmap()

On Wed, Jul 10, 2024 at 03:22:39PM GMT, Liam R. Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
>
> Adding support for a NULL vma means the init_vma_munmap() can be
> initialized for a less error-prone process when calling
> vms_complete_munmap_vmas() later on.
>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
>  mm/mmap.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index a1544a68558e..e2e6b3202c25 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -516,10 +516,12 @@ static inline void init_vma_munmap(struct vma_munmap_struct *vms,
>  		bool unlock)
>  {
>  	vms->vmi = vmi;
> -	vms->vma = vma;
> -	vms->mm = vma->vm_mm;
> -	vms->start = start;
> -	vms->end = end;
> +	if (vma) {
> +		vms->vma = vma;
> +		vms->mm = vma->vm_mm;
> +		vms->start = start;
> +		vms->end = end;
> +	}

Why not store start/end even if !vma? And shouldn't we have an else clause
to make sure these are initialised in this case too?

I mean also we could have vms->vma = vma outside of this clause to so it
looks something like:

	vms->vma = vma;
	vms->mm = vma ? vma->vm_mm : NULL;
	vms->start = start;
	vms->end = end;

>  	vms->unlock = unlock;
>  	vms->uf = uf;
>  	vms->vma_count = 0;
> @@ -2950,11 +2952,11 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>
>  	/* Find the first overlapping VMA */
>  	vma = vma_find(&vmi, end);
> +	init_vma_munmap(&vms, &vmi, vma, addr, end, uf, /* unlock = */ false);
>  	if (vma) {
>  		mt_init_flags(&mt_detach, vmi.mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
>  		mt_on_stack(mt_detach);
>  		mas_init(&mas_detach, &mt_detach, /* addr = */ 0);
> -		init_vma_munmap(&vms, &vmi, vma, addr, end, uf, /* unlock = */ false);
>  		/* Prepare to unmap any existing mapping in the area */
>  		if (vms_gather_munmap_vmas(&vms, &mas_detach))
>  			return -ENOMEM;
> --
> 2.43.0
>

I really like this approach in general though!