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Howlett" To: David Hildenbrand Cc: Kalesh Singh , Lorenzo Stoakes , akpm@linux-foundation.org, minchan@kernel.org, kernel-team@android.com, android-mm@google.com, 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: Mail-Followup-To: "Liam R. 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R. Howlett wrote: > > * Kalesh Singh [250904 13:51]: > > > On Thu, Sep 4, 2025 at 10:42=E2=80=AFAM David Hildenbrand wrote: > > > >=20 > > > > 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(st= ruct 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 >=3D sysctl_max_map_count - 3= ) > > > > > > > > + if (exceeds_max_map_count(current->mm, 4)) > > > > > > > > return -ENOMEM; > > > > > > >=20 > > > > > > > In my version this would be: > > > > > > >=20 > > > > > > > if (map_count_capacity(current->mm) < 4) > > > > > > > return -ENOMEM; > > > > > > >=20 > > > > > >=20 > > > > > > Someone could write map_count_capacity(current->mm) <=3D 4 and = reintroduce > > > > > > what this is trying to solve. And with the way it is written i= n this > > > > > > patch, someone could pass in the wrong number. > > > > >=20 > > > > > Right, but I think 'capacity' is pretty clear here, if the caller= does something > > > > > silly then that's on them... > > > > >=20 > > > > > >=20 > > > > > > I'm not sure this is worth doing. There are places we allow th= e count > > > > > > to go higher. > > > > >=20 > > > > > ...But yeah, it's kinda borderline as to how useful this is. > > > > >=20 > > > > > I _do_ however like the 'put map count in one place statically' r= ather than > > > > > having a global, so a minimal version of this could be to just ha= ve a helper > > > > > function that gets the sysctl_max_map_count, e.g.: > > > > >=20 > > > > > if (current->mm->mmap_count >=3D max_map_count() - 3) > > > >=20 > > > > I enjoy seeing sysctl_max_map_count hidden. But map_count_capacity(= ) is > > > > even more readable, so I like it. > > > >=20 > > > > 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. > > > >=20 > > > > I also don't completely like "map_count" (I know, I know, we call i= t > > > > like that in structures), because it reminds me of the mapcount ... > > > > talking somehow about "vmas" would be quite clear. > > >=20 > > > Thanks David, my original implementation started with vma_limit() :). > > > Maybe something like vma_count_remaining() ? > >=20 > > 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. > >=20 > > vma_headroom() ? > > additional_vma_space() ? >=20 > VMA space might be interpreted as VA space. Fair enough. >=20 > I think basing it on "vma_count" would be good. >=20 > vma_count_capacity() >=20 > vma_count_headroom() >=20 > vma_count_remaining() headroom or remaining have my vote as the clearest. >=20 > vma_count_avail() >=20 > vma_count_left() >=20