Re: [PATCH v10 11/37] mm: page_alloc: move prep_compound_page before post_alloc_hook

[Lorenzo Stoakes <ljs@kernel.org>]

On Mon, Jun 08, 2026 at 04:36:20AM -0400, Michael S. Tsirkin wrote:
> Move prep_compound_page() before post_alloc_hook() in prep_new_page().
>
> The next patch adds a folio_zero_user() call to post_alloc_hook(),
> which uses folio_nr_pages() to determine how many pages to zero.
> Without compound metadata set up first, folio_nr_pages() returns 1
> for higher-order allocations, so only the first page would be zeroed.
>
> All other operations in post_alloc_hook() (arch_alloc_page, KASAN,
> debug, page owner, etc.) use raw page pointers with explicit order
> counts and are unaffected by this reordering.

I'd put this justification for why this is safe above the 'next patch' stuff.

>
> Also reorder compaction_alloc_noprof() for consistency. Compaction
> currently passes USER_ADDR_NONE so folio_zero_user() is not called

Yeah again, this 'just so' stuff of when or when not an address is passed is
concerning.

> there, but keeping the same ordering avoids a future tripping hazard.

No functional changes or functional changes? If none then say so.

>
> Reviewed-by: Gregory Price <gourry@gourry.net>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Assisted-by: Claude:claude-opus-4-6
> Assisted-by: cursor-agent:GPT-5.4-xhigh
> ---
>  mm/compaction.c | 4 ++--
>  mm/page_alloc.c | 4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 72684fe81e83..4336e433c99b 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1849,10 +1849,10 @@ static struct folio *compaction_alloc_noprof(struct folio *src, unsigned long da
>  		set_page_private(&freepage[size], start_order);
>  	}
>  	dst = (struct folio *)freepage;
> -	post_alloc_hook(&dst->page, order, __GFP_MOVABLE, USER_ADDR_NONE);
> -	set_page_refcounted(&dst->page);
>  	if (order)
>  		prep_compound_page(&dst->page, order);
> +	post_alloc_hook(&dst->page, order, __GFP_MOVABLE, USER_ADDR_NONE);
> +	set_page_refcounted(&dst->page);
>  	cc->nr_freepages -= 1 << order;
>  	cc->nr_migratepages -= 1 << order;
>  	return page_rmappable_folio(&dst->page);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0943ab724032..4676fd49819e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1874,11 +1874,11 @@ static void prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags
>  							unsigned int alloc_flags,
>  							unsigned long user_addr)
>  {
> -	post_alloc_hook(page, order, gfp_flags, user_addr);
> -
>  	if (order && (gfp_flags & __GFP_COMP))
>  		prep_compound_page(page, order);
>
> +	post_alloc_hook(page, order, gfp_flags, user_addr);
> +
>  	/*
>  	 * page is set pfmemalloc when ALLOC_NO_WATERMARKS was necessary to
>  	 * allocate the page. The expectation is that the caller is taking
> --
> MST
>

Thanks, Lorenzo