Re: [PATCH v2 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page()

[Huang, Ying]

Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:

> Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(), which
> checks whether the given zone contains holes, and uses pfn_to_online_page()
> to validate if the start pfn is online and valid, as well as using pfn_valid()
> to validate the end pfn.
>
> However, the __pageblock_pfn_to_page() function may return non-NULL even
> if the end pfn of a pageblock is in a memory hole in some situations. For
> example, if the pageblock order is MAX_ORDER, which will fall into 2
> sub-sections, and the end pfn of the pageblock may be hole even though
> the start pfn is online and valid.
>
> This did not break anything until now, but the zone continuous is fragile
> in this possible scenario. So as previous discussion[1], it is better to
> add some comments to explain this possible issue in case there are some
> future pfn walkers that rely on this.
>
> [1] https://lore.kernel.org/all/87r0sdsmr6.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
>
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>

Looks good to me!  Thanks!

Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

> ---
> Changes from v1:
>  - Update the comments per Ying and Mike, thanks.
> ---
>  mm/page_alloc.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6457b64fe562..9756d66f471c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1502,6 +1502,13 @@ void __free_pages_core(struct page *page, unsigned int order)
>   * interleaving within a single pageblock. It is therefore sufficient to check
>   * the first and last page of a pageblock and avoid checking each individual
>   * page in a pageblock.
> + *
> + * Note: the function may return non-NULL even if the end pfn of a pageblock
> + * is in a memory hole in some situations. For example, if the pageblock
> + * order is MAX_ORDER, which will fall into 2 sub-sections, and the end pfn
> + * of the pageblock may be hole even though the start pfn is online and valid.
> + * This did not break anything until now, but be careful about this possible
> + * issue when checking whether all pfns of a pageblock are valid.
>   */
>  struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
>  				     unsigned long end_pfn, struct zone *zone)