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	Mon, 10 Mar 2025 16:30:49 +0000
Date: Mon, 10 Mar 2025 16:30:26 +0000
From: Matthew Wilcox <willy@infradead.org>
To: Zi Yan <ziy@nvidia.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>,
	Ryan Roberts <ryan.roberts@arm.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	David Hildenbrand <david@redhat.com>,
	Yang Shi <yang@os.amperecomputing.com>,
	Miaohe Lin <linmiaohe@huawei.com>,
	Kefeng Wang <wangkefeng.wang@huawei.com>,
	Yu Zhao <yuzhao@google.com>, John Hubbard <jhubbard@nvidia.com>,
	Baolin Wang <baolin.wang@linux.alibaba.com>,
	linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
	Kairui Song <kasong@tencent.com>
Subject: Re: [PATCH v10 2/8] mm/huge_memory: add two new (not yet used)
 functions for folio_split()
Message-ID: <Z88ToirSWa_meevw@casper.infradead.org>
References: <20250307174001.242794-1-ziy@nvidia.com>
 <20250307174001.242794-3-ziy@nvidia.com>
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In-Reply-To: <20250307174001.242794-3-ziy@nvidia.com>

On Fri, Mar 07, 2025 at 12:39:55PM -0500, Zi Yan wrote:
> +	for (index = new_nr_pages; index < nr_pages; index += new_nr_pages) {
> +		struct page *head = &folio->page;
> +		struct page *new_head = head + index;
> +
> +		/*
> +		 * Careful: new_folio is not a "real" folio before we cleared PageTail.
> +		 * Don't pass it around before clear_compound_head().
> +		 */
> +		struct folio *new_folio = (struct folio *)new_head;
[...]
> +		/* ->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);
> +		new_head->mapping = head->mapping;
> +		new_head->index = head->index + index;

Why are you using new_head->mapping and ->index instead of new_folio?