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Date: Mon, 5 Feb 2024 10:31:28 +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] mm: hugetlb: fix hugetlb allocation failure when
 handling freed or in-use hugetlb
Message-ID: <ZcCq8L6KpmAs11py@tiehlicka>
References: <b2e6ce111400670d8021baf4d7ac524ae78a40d5.1707105047.git.baolin.wang@linux.alibaba.com>
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In-Reply-To: <b2e6ce111400670d8021baf4d7ac524ae78a40d5.1707105047.git.baolin.wang@linux.alibaba.com>

On Mon 05-02-24 11:54:17, Baolin Wang wrote:
> When handling the freed hugetlb or in-use hugetlb, we should ignore the
> failure of alloc_buddy_hugetlb_folio() to dissolve the old hugetlb successfully,
> since we did not use the new allocated hugetlb in this 2 cases.
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  mm/hugetlb.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 9d996fe4ecd9..212ab331d355 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3042,9 +3042,8 @@ static int alloc_and_dissolve_hugetlb_folio(struct hstate *h,
>  	 * 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);
> +	if (new_folio)
> +		__prep_new_hugetlb_folio(h, new_folio);

Is there any reason why you haven't moved the allocation to the only
branch that actually needs it? I know that we hold hugetlb lock but you
could have easily dropped the lock, allocate a page and then goto retry.
This would actually save an allocation.

Something like this:

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ed1581b670d4..db5f72b94422 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3029,21 +3029,9 @@ 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)) {
@@ -3073,6 +3061,15 @@ 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
@@ -3100,9 +3097,11 @@ 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;
 }
-- 
Michal Hocko
SUSE Labs