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: <A10312E5-F4CE-4CBA-B7CB-D435DFEA496F@nvidia.com>
Date: Mon, 10 Mar 2025 12:39:57 -0400
From: Zi Yan <ziy@...dia.com>
To: Matthew Wilcox <willy@...radead.org>
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 10 Mar 2025, at 12:30, Matthew Wilcox wrote:

> 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?

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?

Best Regards,
Yan, Zi