Re: [PATCH 1/2] mm/hugetlb: account migration target folio in per-node NR_HUGETLB vmstat

[Joshua Hahn]

On Mon, 21 Sep 2026 17:12:34 +0800 Hongfu Li <hongfu.li@xxxxxxxxx> wrote:

Hi Hongfu,

Thanks for this fix. From the cover letter I was frowning because I was
thinking to myself "surely there's no way hugetlb accounting is this
broken..." but it seems like indeed it is. 

> From: Hongfu Li <lihongfu@xxxxxxxxxx>
> 
> The NR_HUGETLB vmstat counter is maintained per folio's node: incremented
> when a huge page is handed to a user via hugetlb_alloc_folio() and
> decremented when it is returned to the pool via free_huge_folio().
> 
> A folio obtained by alloc_hugetlb_folio_nodemask() never goes through
> hugetlb_alloc_folio(), so it is never accounted, while its free always
> is.

That's pretty scary! Glad that you caught it here.

> For a migration target this means the target node gets no matching
> increment for the decrement on the old node, so the global nr_hugetlb in
> /proc/vmstat drops by nr_pages for each migration. The same asymmetry
> affects the failed migration path, which frees the target again right
> away, and the temporary folio hugetlb_mfill_atomic_pte() takes from the
> same helper.
> 
> alloc_hugetlb_folio_reserve(), used to preallocate the memfd page cache
> folios, has the same asymmetry: the folio is handed to a user without
> being accounted, while its free is accounted through free_huge_folio().
> 
> Account the folio where it is obtained, in alloc_hugetlb_folio_nodemask()
> and alloc_hugetlb_folio_reserve(), so that the increment pairs with the
> decrement in free_huge_folio(): a successful migration hands the folio
> to a user, a failed one frees it again.

The changes look good to me, and I was able to reproduce the issue on
my host and confirm that after this change, the missing charge is
fully accounted for.
Tested-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
Reviewed-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>

Again, I'm super surprised that a problem this big has gone unnoticed
for so long. Thanks again for working on this fix!

I hope you have a great day : -)
Joshua

> Fixes: 05d4532b60e3 ("memcg/hugetlb: add hugeTLB counters to memcg")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Hongfu Li <lihongfu@xxxxxxxxxx>
> ---