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
