Re: [PATCH] mm: nommu: add sysctl_max_map_count() check for do_mmap()

[Andrew Morton]

On Mon, 06 Jul 2026 09:19:02 +0900 Hajime Tazaki <thehajime@xxxxxxxxx> wrote:

> 
> On Mon, 06 Jul 2026 07:15:12 +0900,
> Andrew Morton wrote:
> > 
> > On Thu,  2 Jul 2026 10:28:30 +0900 Hajime Tazaki <thehajime@xxxxxxxxx> wrote:
> > 
> > > The sysctl variable vm.max_map_count (sysctl_max_map_count) is not
> > > expose under !MMU case but used it wit the default vaule
> > > DEFAULT_MAX_MAP_COUNT as a limit of count.  This is currently used when
> > > a vma entry is split into two chunks (split_vma()) but not used when
> > > allocated (do_mmap()).  As a result, even if users request a large
> > > number of allocate memory, it will keep allocating until OOM happens.
> > > 
> > > This commit introduces a check at the begining of do_mmap in nommu.c to
> > > prevent this situation.
> > > 
> > > This is detected with a LTP (Linux Test Project) test, which linked
> > > below.
> > 
> > AI review flagged a couple of issues in the current code, one quite
> > serious:
> > 
> > https://sashiko.dev/#/patchset/20260702012830.667205-1-thehajime@xxxxxxxxx
> 
> I will follow up with another patch for the issue detected here.

Great, thanks.

> btw, I found (and sashiko also detected) several typos for this
> particular patch.  Can I send v2 patch with the additional fix, or do
> you prefer that I'll only send the additional patch ?

Ah.  Well.  Confession.  I actually quietly fed your changelog through
Gemini to fix up a few things, resulting in

: The sysctl variable vm.max_map_count (sysctl_max_map_count) is not exposed
: under !MMU configurations, but its default value (DEFAULT_MAX_MAP_COUNT)
: is still used as a allocation limit.  Currently, this limit is enforced
: when a VMA entry is split into two chunks (split_vma()), but it is not
: checked during initial allocation (do_mmap()).  As a result, if a user
: requests a large number of memory allocations, the system will continue
: allocating until it hits an Out-Of-Memory (OOM) condition.
: 
: This commit introduces a check at the beginning of do_mmap() in nommu.c to
: prevent this situation.
: 
: This issue was detected using the Linux Test Project (LTP) test linked
: below.

I did this "quietly" because it feels a bit rude telling people that
their English isn't great ;)

Lots of non-English speakers appear to be using LLMs on their
changelogs nowadays and as long as they double-check the result, I
think it's working very well!