Re: [PATCH] mm/hugetlb: restore failed global reservations to subpool in alloc_hugetlb_folio

[Joshua Hahn]

On Mon, 13 Jul 2026 19:50:08 +0800 Song Hu <husong@xxxxxxxxxx> wrote:

Hi Song, thank you for the patch.

> When hugetlb_alloc_folio() fails, alloc_hugetlb_folio() only rolls back
> spool->used_hpages in the out_subpool_put path when gbl_chg == 0. For
> gbl_chg > 0 (e.g. a size= hugetlbfs mount), hugepage_subpool_get_pages()
> has already incremented used_hpages, but the error path skips the
> rollback, so each failed fault permanently leaks one used_hpage until
> the subpool is exhausted and hugepage_subpool_get_pages() itself fails.
> 
> Decrement used_hpages for the gbl_chg > 0 case too, mirroring the
> hugetlb_reserve_pages() fix.

So something is clearly wrong with this codepath here; there are now 4
competing fixes in the mailing list currently being discussed [1] [2] [3]
including this one and they all do things slightly differently.
Let's please agree on what the correct solution is, 
I've CC-ed the authors of those 3 other solutions to discuss here. 

> Fixes: a833a693a490 ("mm: hugetlb: fix incorrect fallback for subpool")
> Signed-off-by: Song Hu <husong@xxxxxxxxxx>
> ---
>  mm/hugetlb.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index d6c812d1857b..8413ec92d836 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3073,6 +3073,19 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
>  	if (map_chg && !gbl_chg) {
>  		gbl_reserve = hugepage_subpool_put_pages(spool, 1);
>  		hugetlb_acct_memory(h, -gbl_reserve);
> +	} else if (map_chg && gbl_chg > 0 && spool) {
> +		/*
> +		 * Restore used_hpages for the globally-requested page that
> +		 * hugepage_subpool_get_pages() counted against the subpool's
> +		 * maximum, but which we failed to back from the global pool.
> +		 * Mirrors the fix in hugetlb_reserve_pages() (1d3f9bb4c8af).
> +		 */
> +		unsigned long flags;
> +
> +		spin_lock_irqsave(&spool->lock, flags);
> +		if (spool->max_hpages != -1)
> +			spool->used_hpages -= gbl_chg;
> +		unlock_or_release_subpool(spool, flags);

Why are we unlocking or releasing the subpool here? 

>  	}
>  
>  out_end_reservation:
> -- 
> 2.43.0

Thanks again for the patch,
Joshua

[1] https://lore.kernel.org/all/20260708-hugetlb-alloc-failure-fixes-v2-2-c7f27cbb462b@xxxxxxxxxx/
[2] https://lore.kernel.org/linux-mm/20260428113037.88766-2-enderaoelyther@xxxxxxxxx/
[3] https://lore.kernel.org/linux-mm/20260515202902.461539-1-devnexen@xxxxxxxxx/