On 09/14/22 13:58, Andrew Morton wrote:

On Wed, 14 Sep 2022 10:23:05 -0700 Mike Kravetz [off-list ref] wrote:

quoted

On 09/13/22 12:54, Doug Berger wrote:

quoted

When demoting a hugepage to a smaller order, the number of pages
added to the target hstate will be the size of the large page
divided by the size of the smaller page.

Fixes: 8531fc6f52f5 ("hugetlb: add hugetlb demote page support")
Signed-off-by: Doug Berger <opendmb@gmail.com>
---
 mm/hugetlb.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e070b8593b37..79949893ac12 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c

@@ -3472,7 +3472,8 @@ static int demote_free_huge_page(struct hstate *h, struct page *page)
 	 * based on pool changes for the demoted page.
 	 */
 	h->max_huge_pages--;
-	target_hstate->max_huge_pages += pages_per_huge_page(h);
+	target_hstate->max_huge_pages += pages_per_huge_page(h) /
+					 pages_per_huge_page(target_hstate);
 
 	return rc;
 }

This has already been fixed here,

https://lore.kernel.org/linux-mm/20220823030209.57434-2-linmiaohe@huawei.com/ (local)

Neither version tells us the user-visible runtime effects of the change :(

Sorry, I should have pushed harder on this with Miaohe's patch.

There are no user-visible runtime effects.  In fact, this change really causes
no functional change (unless I am mistaken and Miaohe did not correct me).
max_huge_pages is not used again until it is reset.  See my explanation at:
https://lore.kernel.org/linux-mm/YvwfvxXewnZpHQcz@monkey/ (local)
-- 
Mike Kravetz