On Fri, Feb 19, 2021 at 10:56:42AM +0100, Michal Hocko wrote:

OK, this should work but I am really wondering whether it wouldn't be
just simpler to replace the old page by a new one in the free list
directly. Or is there any reason we have to go through the generic
helpers path? I mean something like this

	new_page = alloc_fresh_huge_page();
	if (!new_page)
		goto fail;
	spin_lock(hugetlb_lock);
	if (!PageHuge(old_page)) {
		/* freed from under us, nothing to do */ 
		__update_and_free_page(new_page);
		goto unlock;
	}
	list_del(&old_page->lru);
	__update_and_free_page(old_page);
	__enqueue_huge_page(new_page);
unlock:
	spin_unlock(hugetlb_lock);

This will require to split update_and_free_page and enqueue_huge_page to
counters independent parts but that shouldn't be a big deal. But it will
also protect from any races. Not an act of beauty but seems less hackish
to me.

Yes, I think this would to the trick, and it is race-free.
Let me play with it a bit and see what I can come up with.

Thanks for the valuable insight.

-- 
Oscar Salvador
SUSE L3