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Date: Tue, 6 Feb 2024 11:09:18 +0100
From: Michal Hocko <mhocko@suse.com>
To: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: akpm@linux-foundation.org, muchun.song@linux.dev, osalvador@suse.de,
	david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] mm: hugetlb: improve the handling of hugetlb
 allocation failure for freed or in-use hugetlb
Message-ID: <ZcIFTqgQThcSdOHl@tiehlicka>
References: <62890fd60b1ecd5bf1cdc476c973f60fe37aa0cb.1707181934.git.baolin.wang@linux.alibaba.com>
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In-Reply-To: <62890fd60b1ecd5bf1cdc476c973f60fe37aa0cb.1707181934.git.baolin.wang@linux.alibaba.com>

On Tue 06-02-24 11:08:11, Baolin Wang wrote:
> alloc_and_dissolve_hugetlb_folio() preallocates a new hugetlb page before
> it takes hugetlb_lock. In 3 out of 4 cases the page is not really used and
> therefore the newly allocated page is just freed right away. This is
> wasteful and it might cause pre-mature failures in those cases.
> 
> Address that by moving the allocation down to the only case (hugetlb
> page is really in the free pages pool). We need to drop hugetlb_lock
> to do so and therefore need to recheck the page state after regaining
> it.
> 
> The patch is more of a cleanup than an actual fix to an existing
> problem. There are no known reports about pre-mature failures.
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>

Acked-by: Michal Hocko <mhocko@suse.com>
Thanks!

> ---
> Changes from v2；
>  - Update the commit message suggested by Michal.
>  - Remove unnecessary comments.
> Changes from v1:
>  - Update the suject line per Muchun.
>  - Move the allocation into the free hugetlb handling branch per Michal.
> ---
>  mm/hugetlb.c | 32 ++++++++++++++++----------------
>  1 file changed, 16 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 9d996fe4ecd9..a05507a2143f 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3031,21 +3031,9 @@ static int alloc_and_dissolve_hugetlb_folio(struct hstate *h,
>  {
>  	gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
>  	int nid = folio_nid(old_folio);
> -	struct folio *new_folio;
> +	struct folio *new_folio = NULL;
>  	int ret = 0;
>  
> -	/*
> -	 * Before dissolving the folio, we need to allocate a new one for the
> -	 * pool to remain stable.  Here, we allocate the folio and 'prep' it
> -	 * by doing everything but actually updating counters and adding to
> -	 * the pool.  This simplifies and let us do most of the processing
> -	 * under the lock.
> -	 */
> -	new_folio = alloc_buddy_hugetlb_folio(h, gfp_mask, nid, NULL, NULL);
> -	if (!new_folio)
> -		return -ENOMEM;
> -	__prep_new_hugetlb_folio(h, new_folio);
> -
>  retry:
>  	spin_lock_irq(&hugetlb_lock);
>  	if (!folio_test_hugetlb(old_folio)) {
> @@ -3075,6 +3063,16 @@ static int alloc_and_dissolve_hugetlb_folio(struct hstate *h,
>  		cond_resched();
>  		goto retry;
>  	} else {
> +		if (!new_folio) {
> +			spin_unlock_irq(&hugetlb_lock);
> +			new_folio = alloc_buddy_hugetlb_folio(h, gfp_mask, nid,
> +							      NULL, NULL);
> +			if (!new_folio)
> +				return -ENOMEM;
> +			__prep_new_hugetlb_folio(h, new_folio);
> +			goto retry;
> +		}
> +
>  		/*
>  		 * Ok, old_folio is still a genuine free hugepage. Remove it from
>  		 * the freelist and decrease the counters. These will be
> @@ -3102,9 +3100,11 @@ static int alloc_and_dissolve_hugetlb_folio(struct hstate *h,
>  
>  free_new:
>  	spin_unlock_irq(&hugetlb_lock);
> -	/* Folio has a zero ref count, but needs a ref to be freed */
> -	folio_ref_unfreeze(new_folio, 1);
> -	update_and_free_hugetlb_folio(h, new_folio, false);
> +	if (new_folio) {
> +		/* Folio has a zero ref count, but needs a ref to be freed */
> +		folio_ref_unfreeze(new_folio, 1);
> +		update_and_free_hugetlb_folio(h, new_folio, false);
> +	}
>  
>  	return ret;
>  }
> -- 
> 2.39.3
> 

-- 
Michal Hocko
SUSE Labs