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

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Message-ID: <4chxl7uxr4exy2z2dcshxla3c5nzzo2tbnelsbbky7pdzrih6a@hzfnpbenfmub>
Date: Thu, 4 Sep 2025 14:49:47 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Kalesh Singh <kaleshsingh@...gle.com>
Cc: David Hildenbrand <david@...hat.com>,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        akpm@...ux-foundation.org, minchan@...nel.org, kernel-team@...roid.com,
        android-mm@...gle.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

* Kalesh Singh <kaleshsingh@...gle.com> [250904 13:51]:
> On Thu, Sep 4, 2025 at 10:42 AM David Hildenbrand <david@...hat.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() ?

Yes, reducing this confusion would very much be helpful.  In fact, if
you put it in its own function we could change the actual name with
lower impact.  map_count vs mapcount is annoying.

vma_headroom() ?
additional_vma_space() ?

Maybe David would like:
remedy_vma_space() which would be !poison_vma_space().. that's pretty
clear.. :)

Cheers,
Liam