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

[Kalesh Singh]

On Thu, Sep 4, 2025 at 10:33 AM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
>
> 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.

Hi Liam,

I still think there is value to this as it's lot less likely to get
the common case incorrectly:

if (!map_count_capacity(mm))
        return -ENOMEM;

It also facilitate us adding the asserts as Pedro suggested (excluding
the munmap() case.

>
> 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

Thanks Lorenzo, will do.

-- Kalesh

>
> Cheers!
>
> >
> > Thanks,
> > Liam