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

[Kalesh Singh <kaleshsingh@google.com>]

On Thu, Sep 4, 2025 at 10:42 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 04.09.25 19:33, Lorenzo Stoakes 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.
> >
> > 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)
>
> I enjoy seeing sysctl_max_map_count hidden. But map_count_capacity() is
> even more readable, so I like it.
>
> I don't complete like the "capacity" term, but I cannot think of
> something better right now. Maybe something around "free" or
> "remaining", not sure.
>
> I also don't completely like "map_count" (I know, I know, we call it
> like that in structures), because it reminds me of the mapcount ...
> talking somehow about "vmas" would be quite clear.

Thanks David, my original implementation started with vma_limit() :).
Maybe something like vma_count_remaining() ?

-- Kalesh
>
> Anyhow, just as an inspiration my 2 cents ...
>
> --
> Cheers
>
> David / dhildenb
>