On Thu, Aug 28, 2025 at 12:01:13AM +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 comment to describe the initialization order: again,
> just like prep_compound_page().
Wow this is great, thank you for putting a quality comment for this and
thinking of this :)
We have too much 'special case you just have to know' stuff sitting around,
so this kind of thing is always great to see.
>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
LGTM, so:
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> ---
> mm/mm_init.c | 15 +++++++--------
> 1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 5c21b3af216b2..df614556741a4 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1091,6 +1091,12 @@ static void __ref memmap_init_compound(struct page *head,
> unsigned long pfn, end_pfn = head_pfn + nr_pages;
> unsigned int order = pgmap->vmemmap_shift;
>
> + /*
> + * We have to initialize the pages, including setting up page links.
> + * prep_compound_page() does not take care of that, so instead we
> + * open-code prep_compound_page() so we can take care of initializing
> + * the pages in the same go.
> + */
> __SetPageHead(head);
> for (pfn = head_pfn + 1; pfn < end_pfn; pfn++) {
> struct page *page = pfn_to_page(pfn);
> @@ -1098,15 +1104,8 @@ 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
>
