Re: [PATCH v2 3/4] mm: hugetlb: Fix subpool usage leak on allocation failure

[Joshua Hahn <joshua.hahnjy@gmail.com>]

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On Wed, 9 Sep 2026 15:54:59 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> On Wed, 09 Sep 2026 14:49:28 -0700 Ackerley Tng via B4 Relay <devnull+ackerleytng.google.com@kernel.org> wrote:
> 
> > From: Ackerley Tng <ackerleytng@google.com>
> > 
> > When folio allocation fails early (e.g. buddy allocation failure or
> > cgroup charging failure) and a reservation was not used (meaning an
> > unreserved global page was needed), the subpool page acquired during the
> > allocation attempt must still be returned.
> > 
> > Currently, the subpool cleanup error path only returns the page to the
> > subpool if a reservation was used. If no reservation was used, it skips
> > releasing the page back to the subpool, permanently leaking the subpool's
> > used pages counter.
> > 
> > With subpools now always tracking used pages, always release the page
> > back to the subpool whenever a subpool page was acquired.
> > 
> > Opportunistically rename the local variables tracking global reservations
> > needed and global reservations returned. This clarifies the accounting:
> > a value of zero for needed global reservations indicates an existing
> > reservation satisfies the allocation, while a non-zero value indicates
> > new global pages are required.
> > 
> > Adjust global reservations using the difference between reservations
> > needed and reservations returned to properly handle races where concurrent
> > threads interact with the same subpool.
> 
> This one conflicts with "mm/hugetlb: fix max-only subpool accounting on
> alloc_hugetlb_folio failure". 
> https://lore.kernel.org/20260428113037.88766-2-enderaoelyther@gmail.com.
> 
> 
> > @@ -3074,13 +3074,10 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
> >  	return folio;
> >  
> >  out_subpool_put:
> > -	/*
> > -	 * put page to subpool iff the quota of subpool's rsv_hpages is used
> > -	 * during hugepage_subpool_get_pages.
> > -	 */
> > -	if (map_chg && !gbl_chg) {
> > -		gbl_reserve = hugepage_subpool_put_pages(spool, 1);
> > -		hugetlb_acct_memory(h, -gbl_reserve);
> > +	if (map_chg) {
> > +		long gbl_resv_put = hugepage_subpool_put_pages(spool, 1);
> > +
> > +		hugetlb_acct_memory(h, gbl_resv_get - gbl_resv_put);
> >  	}
> >  
> >  out_end_reservation:
> 
> Here.
> 
> I presently have Zhao Li's patch staged in mm.git's
> mm-hotfixes-unstable branch, awaiting review.
> 
> What to do?

Hi Andrew, I hope you are having a good day!

Yeah, I also noticed this too [1]. There are actually 4 proposed
solutions for this in the mailing list which is a good indication that
something is wrong here : -) But unfortunately it seems like there was 
no resolution the last time I pointed this out : -(

I CC-ed all the authors previously on that chain but wasn't able to
get a response, maybe we'll give it another shot here?

Ackerley, Zhao, David, Song, hopefully we can align here.

Personally I'm partial to Ackerley's approach here since I think the
get/put semantics that he proposes is the easiest to understand for me.

It would be great if we could sort this out since I think it's been
a while since we've had all of these proposals in flight.

Thank you everyone!
Joshua

[1] https://lore.kernel.org/all/20260713144557.3845941-1-joshua.hahnjy@gmail.com/