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Howlett" To: Kalesh Singh Cc: Lorenzo Stoakes , akpm@linux-foundation.org, minchan@kernel.org, kernel-team@android.com, android-mm@google.com, David Hildenbrand , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jann Horn , Pedro Falcato , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: centralize and fix max map count limit checking Message-ID: <5n37yzto6ylq2a5lnxwcr7osygwkvlbv35fkilkhfc5eqhdqca@iji543qzcy2t> Mail-Followup-To: "Liam R. 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[250904 13:44]: > On Thu, Sep 4, 2025 at 10:33=E2=80=AFAM 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 v= ma_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 >=3D 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) <=3D 4 and reintr= oduce > > > what this is trying to solve. And with the way it is written in this > > > patch, someone could pass in the wrong number. >=20 > Hi Liam, >=20 > I still think there is value to this as it's lot less likely to get > the common case incorrectly: >=20 > if (!map_count_capacity(mm)) > return -ENOMEM; >=20 > It also facilitate us adding the asserts as Pedro suggested (excluding > the munmap() case. And munmap() callers case, I guess. There is still the possibility of us failing to unmap after a split and having shot ourselves, so just don't assert that in the case of the failure and recovery path (ie exit_mmap()). >=20 > > > > Right, but I think 'capacity' is pretty clear here, if the caller does = something > > silly then that's on them... Turns out it's on us, not them :) > > > > > > > > I'm not sure this is worth doing. There are places we allow the coun= t > > > 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 h= elper > > function that gets the sysctl_max_map_count, e.g.: > > > > if (current->mm->mmap_count >=3D max_map_count() - 3) > > > > etc. etc. > > > > > > > > Certainly fix the brk < to be <=3D and any other calculations, but th= e > > > rest seem okay as-is to me. The only real way to be sure we don't ca= use > > > a bug in the future is to have better testing. > > > > Speaking of testing - Kalesh - do make sure to test the VMA tests to ma= ke sure > > this doesn't break those - they live in tools/testing/vma and you just = have to > > do make && ./vma Bonus points if you can add some tests for it! Thanks, Liam