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

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Message-ID: <mss3ntakl2oddsuimqfigjfvcewmc5ju7l3bychhb7r7i5d7vr@abn227s22rsm>
Date: Thu, 11 Jul 2024 12:04:32 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...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()

* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [240711 10:28]:
> 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;

I'd rather not set it the start/end as it implies there is a start/end
of an unmap operation that won't happen.  I'll just make it an else and
set them to 0.

> 
> >  	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!