linux-kernel - Re: [PATCH] mm: centralize and fix max map count limit checking

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Message-ID: <413ee338-1795-433c-b3d4-72c870488d95@lucifer.local>
Date: Thu, 4 Sep 2025 18:33:46 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Kalesh Singh <kaleshsingh@...gle.com>, akpm@...ux-foundation.org,
        minchan@...nel.org, kernel-team@...roid.com, android-mm@...gle.com,
        David Hildenbrand <david@...hat.com>, Vlastimil Babka <vbabka@...e.cz>,
        Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: centralize and fix max map count limit checking

On Thu, Sep 04, 2025 at 01:22:51PM -0400, Liam R. Howlett wrote:
> > > diff --git a/mm/mremap.c b/mm/mremap.c
> > > index e618a706aff5..793fad58302c 100644
> > > --- a/mm/mremap.c
> > > +++ b/mm/mremap.c
> > > @@ -1040,7 +1040,7 @@ static unsigned long prep_move_vma(struct vma_remap_struct *vrm)
> > >  	 * We'd prefer to avoid failure later on in do_munmap:
> > >  	 * which may split one vma into three before unmapping.
> > >  	 */
> > > -	if (current->mm->map_count >= sysctl_max_map_count - 3)
> > > +	if (exceeds_max_map_count(current->mm, 4))
> > >  		return -ENOMEM;
> >
> > In my version this would be:
> >
> > 	if (map_count_capacity(current->mm) < 4)
> > 		return -ENOMEM;
> >
>
> Someone could write map_count_capacity(current->mm) <= 4 and reintroduce
> what this is trying to solve.  And with the way it is written in this
> patch, someone could pass in the wrong number.

Right, but I think 'capacity' is pretty clear here, if the caller does something
silly then that's on them...

>
> I'm not sure this is worth doing.  There are places we allow the count
> to go higher.

...But yeah, it's kinda borderline as to how useful this is.

I _do_ however like the 'put map count in one place statically' rather than
having a global, so a minimal version of this could be to just have a helper
function that gets the sysctl_max_map_count, e.g.:

if (current->mm->mmap_count >= max_map_count() - 3)

etc. etc.

>
> Certainly fix the brk < to be <= and any other calculations, but the
> rest seem okay as-is to me.  The only real way to be sure we don't cause
> a bug in the future is to have better testing.

Speaking of testing - Kalesh - do make sure to test the VMA tests to make sure
this doesn't break those - they live in tools/testing/vma and you just have to
do make && ./vma

Cheers!

>
> Thanks,
> Liam