Re: [PATCH v2 4/5] mm/page_alloc: make sure tail_page->private is zero at page free time

[Vlastimil Babka (SUSE)]

On 7/3/26 15:47, Zi Yan wrote:
> Any code using tail_page->private of a folio, a compound page or a
> high-order page is supposed to reset it after use, otherwise ->private data
> can leak to new page user and cause unexpected issues. Add a bad_page()
> check at page free path for it.
> 
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>

Nit:

> ---
>  mm/page_alloc.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 762d9b6bc792f..723a52dfc80f6 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1376,15 +1376,22 @@ static __always_inline bool __free_pages_prepare(struct page *page,
>  #endif
>  		}
>  		for (i = 1; i < (1 << order); i++) {
> +			struct page *tail_page = page + i;
> +
>  			if (compound)
> -				bad += free_tail_page_prepare(page, page + i);
> +				bad += free_tail_page_prepare(page, tail_page);
>  			if (is_check_pages_enabled()) {
> -				if (free_page_is_bad(page + i)) {
> +				if (free_page_is_bad(tail_page)) {
>  					bad++;
>  					continue;
>  				}
>  			}
> -			(page + i)->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
> +			tail_page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
> +			if (is_check_pages_enabled() && tail_page->private) {
> +				bad_page(tail_page, "nonzero private");
> +				bad++;
> +				continue;

It could be moved to the already existing is_check_pages_enabled() block
above. The flags reset doesn't need to happen before the check, no?

> +			}
>  		}
>  	}
>  	if (folio_test_anon(folio)) {
>