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