Re: [PATCH v2] mm/folio: Avoid special handling for order value 0 in folio_set_order

[Matthew Wilcox <willy@infradead.org>]

On Mon, May 15, 2023 at 10:38:09PM +0530, Tarun Sahu wrote:
> @@ -1951,9 +1950,6 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
>  	struct page *p;
>  
>  	__folio_clear_reserved(folio);
> -	__folio_set_head(folio);
> -	/* we rely on prep_new_hugetlb_folio to set the destructor */
> -	folio_set_order(folio, order);
>  	for (i = 0; i < nr_pages; i++) {
>  		p = folio_page(folio, i);
>  
> @@ -1999,6 +1995,9 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
>  		if (i != 0)
>  			set_compound_head(p, &folio->page);
>  	}
> +	__folio_set_head(folio);
> +	/* we rely on prep_new_hugetlb_folio to set the destructor */
> +	folio_set_order(folio, order);

This makes me nervous, as I said before.  This means that
compound_head(tail) can temporarily point to a page which is not marked
as a head page.  That's different from prep_compound_page().  You need to
come up with some good argumentation for why this is safe, and no amount
of testing you do can replace it -- any race in this area will be subtle.