Re: [PATCH v3 3/4] mm/hugetlb: fix NR_HUGETLB accounting for unused surplus folios

[Muchun Song]

> On Nov 6, 2026, at 18:05, Kaitao Cheng <kaitao.cheng@xxxxxxxxx> wrote:
> 
> From: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
> 
> free_huge_folio() unconditionally decrements NR_HUGETLB, but freshly
> allocated surplus folios can be freed before the counter has been
> incremented.
> 
> This happens when alloc_surplus_hugetlb_folio() rejects a fresh folio
> after its second surplus limit check. It also happens when
> gather_surplus_pages() frees folios because a reservation cannot be
> satisfied, or because concurrent pool changes leave excess folios
> after the reservation succeeds.
> 
> These paths leave unmatched decrements, causing HugeTLB usage to be
> underreported and potentially underflowing the counter.
> 
> Increment NR_HUGETLB before calling free_huge_folio() in both paths
> to balance its unconditional decrement.
> 
> Fixes: 05d4532b60e3 ("memcg/hugetlb: add hugeTLB counters to memcg")
> Closes: https://sashiko.dev/#/bug/linux-860a3b8f-0b07-4cd5-a122-aa2288463bee
> Signed-off-by: Kaitao Cheng <chengkaitao@xxxxxxxxxx>

Acked-by: Muchun Song (ByteDance) <muchun.song@xxxxxxxxx>

Thanks.