Re: [PATCH 04/15] mm/hugetlb: account temporary folios separately

[Muchun Song <muchun.song@linux.dev>]

> On Oct 29, 2026, at 04:13, David Hildenbrand (Arm) <david@kernel.org> wrote:
> 
> Hugetlb migration might have to allocate a temporary folio through
> alloc_migrate_hugetlb_folio(). Such folios are currently accounted in
> "nr_huge_pages" but not in "surplus_huge_pages". persistent_huge_pages()
> can mistake them as persistent huge pages, as it merely calculates the
> difference.
> 
> If set_max_huge_pages() shrinks the pool at the wrong time, it will turn
> hugetlb folios into surplus folios until persistent_huge_pages() returns
> the desired number.
> 
> After the migration (successful or unsuccessful) either the new (temporary)
> folio or the old folio gets freed: this will reduce "nr_huge_pages" but
> not "surplus_huge_pages", to undo what was done when allocating a
> temporary folio.
> 
> This can result in having more surplus folios than actually allocated
> folios:
> 
>  HugePages_Total:       0
>  HugePages_Surp:        1
> 
> From then on persistent_huge_pages() returns garbage.
> 
> To fix it, add internal temporary counters and subtract them when
> calculating persistent pool sizes. Update persistent_huge_pages() and add
> persistent_huge_pages_node() to make the code easier to read.
> 
> Count migration targets and rejected surplus allocations as temporary.
> Transfer the per-node temporary count along with the temporary flag on
> migration, such that folio freeing will always do the right thing.
> 
> Take care of calling account_new_hugetlb_folio() only after setting the
> folios to be temporary.
> 
> For now, keep temporary folios in nr_huge_pages, so HugePages_Total and
> Hugetlb continue to include them and overcommit accounting remains
> unchanged. The new temporary counters are internal for now and are not
> exported to userspace.
> 
> Fixes: ab5ac90aecf5 ("mm, hugetlb: do not rely on overcommit limit during migration")
> Cc: stable@kernel.org
> Reported-by: Josef Bacik <josef@toxicpanda.com>
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

Acked-by: Muchun Song <muchun.song@linux.dev>

Thanks.