linux-kernel - Re: [PATCH -next 1/2] mm: compaction: convert to use a folio in isolate_migratepages

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Date:   Mon, 12 Jun 2023 15:41:12 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, jgowans@...zon.com, yuzhao@...gle.com
Subject: Re: [PATCH -next 1/2] mm: compaction: convert to use a folio in
 isolate_migratepages_block()

On Mon, Jun 12, 2023 at 10:34:13PM +0800, Kefeng Wang wrote:
> @@ -959,7 +960,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  				 * Hugepage was successfully isolated and placed
>  				 * on the cc->migratepages list.
>  				 */
> -				low_pfn += compound_nr(page) - 1;
> +				folio = page_folio(page);
> +				low_pfn += folio_nr_pages(folio) - 1;
>  				goto isolate_success_no_list;

Why is this safe?  That is, how do we know that the folio can't be
dissolved under us at this point, then reallocated and hit the
VM_BUG_ON_PGFLAGS(PageTail(page), page) in folio_flags() when we
test folio_test_large()?

> @@ -1132,30 +1137,30 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  			 * and it's on LRU. It can only be a THP so the order
>  			 * is safe to read and it's 0 for tail pages.
>  			 */

^^^ This comment needs to be updated too.

> -		mod_node_page_state(page_pgdat(page),
> -				NR_ISOLATED_ANON + page_is_file_lru(page),
> -				thp_nr_pages(page));
> +		lruvec_del_folio(lruvec, folio);
> +		mod_node_page_state(folio_pgdat(folio),
> +				NR_ISOLATED_ANON + folio_is_file_lru(folio),
> +				folio_nr_pages(folio));

		node_stat_mod_folio()