linux-kernel - Re: [PATCH v2 2/4] mm/migration: remove unneeded lock page and PageMovable check

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Message-ID: <525298ad-5e6a-2f8d-366d-4dcb7eebd093@redhat.com>
Date:   Fri, 29 Apr 2022 12:07:17 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org,
        mike.kravetz@...cle.com, naoya.horiguchi@....com
Cc:     ying.huang@...el.com, hch@....de, dhowells@...hat.com,
        cl@...ux.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/4] mm/migration: remove unneeded lock page and
 PageMovable check

On 25.04.22 15:27, Miaohe Lin wrote:
> When non-lru movable page was freed from under us, __ClearPageMovable must
> have been done. Even if it's not done, ClearPageIsolated here won't hurt
> as page will be freed anyway. So we can thus remove unneeded lock page and
> PageMovable check here.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> ---
>  mm/migrate.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index b779646665fe..0fc4651b3e39 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1093,12 +1093,8 @@ static int unmap_and_move(new_page_t get_new_page,
>  		/* page was freed from under us. So we are done. */
>  		ClearPageActive(page);
>  		ClearPageUnevictable(page);
> -		if (unlikely(__PageMovable(page))) {
> -			lock_page(page);
> -			if (!PageMovable(page))
> -				ClearPageIsolated(page);
> -			unlock_page(page);
> -		}
> +		if (unlikely(__PageMovable(page)))
> +			ClearPageIsolated(page);
>  		goto out;
>  	}

Hm, that code+change raises a couple of questions.

We're doing here the same as in putback_movable_pages(). So I guess the
difference here is that the caller did release the reference while the
page was isolated, while we don't assume the same in
putback_movable_pages().

Shouldn't whoever owned the page have cleared that? IOW, is it even
valid that we see a movable or isolated page here (WARN/BUG?)?

At least for balloon compaction, I remember that __PageMovable() is
properly cleared before freeing it via balloon_page_delete().

Also, I am not sure how reliable that page count check is here: if we'd
have another speculative reference to the page, we might see
"page_count(page) > 1" and not take that path, although the previous
owner released the last reference.

-- 
Thanks,

David / dhildenb