On Thu, Aug 21, 2025 at 10:06:35PM +0200, David Hildenbrand wrote:
Grepping for "prep_compound_page" leaves on clueless how devdax gets its compound pages initialized.
Let's add a comment that might help finding this open-coded prep_compound_page() initialization more easily.
Further, let's be less smart about the ordering of initialization and just perform the prep_compound_head() call after all tail pages were initialized: just like prep_compound_page() does.
No need for a lengthy comment then: again, just like prep_compound_page().
Note that prep_compound_head() already does initialize stuff in page[2] through prep_compound_head() that successive tail page initialization will overwrite: _deferred_list, and on 32bit _entire_mapcount and _pincount. Very likely 32bit does not apply, and likely nobody ever ends up testing whether the _deferred_list is empty.
So it shouldn't be a fix at this point, but certainly something to clean up.
Signed-off-by: David Hildenbrand david@redhat.com
mm/mm_init.c | 13 +++++-------- 1 file changed, 5 insertions(+), 8 deletions(-)
diff --git a/mm/mm_init.c b/mm/mm_init.c index 5c21b3af216b2..708466c5b2cc9 100644 --- a/mm/mm_init.c +++ b/mm/mm_init.c @@ -1091,6 +1091,10 @@ static void __ref memmap_init_compound(struct page *head, unsigned long pfn, end_pfn = head_pfn + nr_pages; unsigned int order = pgmap->vmemmap_shift;
- /*
* This is an open-coded prep_compound_page() whereby we avoid* walking pages twice by initializing them in the same go.*/
While on it, can you also mention that prep_compound_page() is not used to properly set page zone link?
With this
Reviewed-by: Mike Rapoport (Microsoft) rppt@kernel.org
__SetPageHead(head); for (pfn = head_pfn + 1; pfn < end_pfn; pfn++) { struct page *page = pfn_to_page(pfn); @@ -1098,15 +1102,8 @@ static void __ref memmap_init_compound(struct page *head, __init_zone_device_page(page, pfn, zone_idx, nid, pgmap); prep_compound_tail(head, pfn - head_pfn); set_page_count(page, 0);
/** The first tail page stores important compound page info.* Call prep_compound_head() after the first tail page has* been initialized, to not have the data overwritten.*/if (pfn == head_pfn + 1)prep_compound_head(head, order);
- prep_compound_head(head, order);
} void __ref memmap_init_zone_device(struct zone *zone, -- 2.50.1