linux-kernel - Re: [PATCH v10 2/8] mm/huge_memory: add two new (not yet used) functions for folio

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Message-ID: <Z88ar5YS99HsIRYo@casper.infradead.org>
Date: Mon, 10 Mar 2025 17:00:31 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Zi Yan <ziy@...dia.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Ryan Roberts <ryan.roberts@....com>,
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
	David Hildenbrand <david@...hat.com>,
	Yang Shi <yang@...amperecomputing.com>,
	Miaohe Lin <linmiaohe@...wei.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Yu Zhao <yuzhao@...gle.com>, John Hubbard <jhubbard@...dia.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
	Kairui Song <kasong@...cent.com>
Subject: Re: [PATCH v10 2/8] mm/huge_memory: add two new (not yet used)
 functions for folio_split()

On Mon, Mar 10, 2025 at 12:42:06PM -0400, Zi Yan wrote:
> > Because of the “Careful” comment. But new_folio->* should be fine,
> > since it is the same as new_head. So I probably can replace all
> > new_head with new_folio except those VM_BUG_ON_PAGE checks?

Why not also the VM_BUG_ON_PAGE check?  I mean:

> @@ -3364,8 +3364,8 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>  		/* ->mapping in first and second tail page is replaced by other uses */
>  		VM_BUG_ON_PAGE(new_nr_pages > 2 && new_head->mapping != TAIL_MAPPING,
>  			       new_head);

		VM_BUG_ON_PAGE(new_nr_pages > 2 && new_folio->mapping != TAIL_MAPPING, new_head);

(or we could just ditch the assert entirely; it's not all that useful)

> -		new_head->mapping = head->mapping;
> -		new_head->index = head->index + index;
> +		new_folio->mapping = head->mapping;
> +		new_folio->index = head->index + index;

	new_folio->mapping = folio->mapping
	new_folio->index = folio->index +index;

(um, and that index + index looks weird; better name might be just 'i')