linux-kernel - Re: [PATCH 2/3] mm/migrate: Convert isolate_movable

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Date:   Sun, 22 Jan 2023 12:46:34 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        sidhartha.kumar@...cle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] mm/migrate: Convert isolate_movable_page() to use
 folios

On Fri, Jan 20, 2023 at 04:56:21PM -0800, Vishal Moola (Oracle) wrote:
>  int isolate_movable_page(struct page *page, isolate_mode_t mode)
>  {
> +	struct folio *folio = page_folio(page);
>  	const struct movable_operations *mops;
>  
>  	/*
> @@ -71,11 +72,11 @@ int isolate_movable_page(struct page *page, isolate_mode_t mode)
>  	 * the put_page() at the end of this block will take care of
>  	 * release this page, thus avoiding a nasty leakage.
>  	 */
> -	if (unlikely(!get_page_unless_zero(page)))
> +	if (unlikely(!folio_try_get(folio)))

This changes behaviour.  Previously when called on a tail page, the
call failed.  Now it succeeds, getting a ref on something that at
least was the folio head at some point.

If you're going to do this, you need to recheck that the page is still
part of the folio after getting the ref (see gup.c for an example).
But I think we should probably maintain the behaviour of failing on
tail pages.

Maybe something like ...

	if (unlikely(!get_page_unless_zero(page)))
		goto out;
	/* Refcount is zero on tail pages, so we must have a head */
	folio = (struct folio *)page;