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

[Zi Yan <ziy@nvidia.com>]

On 10 Mar 2025, at 12:39, Zi Yan wrote:

> 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?

Something like?

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f06508e4d242..007c927536bb 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3341,8 +3341,8 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
 		 * unreferenced sub-pages of an anonymous THP: we can simply drop
 		 * PG_anon_exclusive (-> PG_mappedtodisk) for these here.
 		 */
-		new_head->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
-		new_head->flags |= (head->flags &
+		new_folio->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
+		new_folio->flags |= (head->flags &
 				((1L << PG_referenced) |
 				 (1L << PG_swapbacked) |
 				 (1L << PG_swapcache) |
@@ -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);
-		new_head->mapping = head->mapping;
-		new_head->index = head->index + index;
+		new_folio->mapping = head->mapping;
+		new_folio->index = head->index + index;

 		/*
 		 * page->private should not be set in tail pages. Fix up and warn once



Best Regards,
Yan, Zi