Re: [PATCH v2] hugetlb: evaluate subpool free state while locked

[jane.chu@oracle.com]

On 7/20/2026 8:52 PM, Yichong Chen wrote:
> unlock_or_release_subpool() drops spool->lock before calling 
> subpool_is_free(). However, subpool_is_free() reads fields that are 
> updated under spool->lock, including count, used_hpages and rsv_hpages. 
> Keep the free-state evaluation under
> 
> 
> unlock_or_release_subpool() drops spool->lock before calling
> subpool_is_free().  However, subpool_is_free() reads fields that are
> updated under spool->lock, including count, used_hpages and rsv_hpages.
> 
> Keep the free-state evaluation under the same lock that protects those
> fields.  The reservation accounting and kfree() calls still happen after
> dropping spool->lock.
> 
> Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> Signed-off-by: Yichong Chen <chenyichong@uniontech.com>
> ---
> v2:
> - Reword the changelog based on Joshua's observation.
> - Drop the Fixes tag because this is not known to cause a user-visible bug.
> - Add Joshua's Reviewed-by.
> 
>   mm/hugetlb.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e319c6a00555..46ab702c0fc0 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -140,12 +140,14 @@ static inline bool subpool_is_free(struct hugepage_subpool *spool)
>   static inline void unlock_or_release_subpool(struct hugepage_subpool *spool,
>   						unsigned long irq_flags)
>   {
> -	spin_unlock_irqrestore(&spool->lock, irq_flags);
> +	bool free_subpool = subpool_is_free(spool);
>   
>   	/* If no pages are used, and no other handles to the subpool
>   	 * remain, give up any reservations based on minimum size and
>   	 * free the subpool */
> -	if (subpool_is_free(spool)) {
> +	spin_unlock_irqrestore(&spool->lock, irq_flags);
> +
> +	if (free_subpool) {
>   		if (spool->min_hpages != -1)
>   			hugetlb_acct_memory(spool->hstate,
>   						-spool->min_hpages);
> -- 
> 2.51.0
> 

Looks good to me.

I also looked at Sashiko's review

  
https://sashiko.dev/#/patchset/20260721035207.1437935-1-chenyichong%40uniontech.com

where it raised a pre-existing issue which, if I'm not mistaken, is not 
an issue?  Because, if spool->max_hpages is not established, 
spool->used_hpages doesn't apply, that is, the subpool tracking is not 
needed for limiting hpage allocation.  To add on that, if 
spool->min_hpages is also not established, subpool is practically not 
needed, and purely for the syntactical reason, when spool->count 
refcount drops to 0, it's okay to free the subpool.

Reviewed-by: Jane Chu <jane.chu@oracle.com>

-jane