Re: [PATCH 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, though the start pfn of a pageblock is valid, it can not always
> guarantee the end pfn of the pageblock is also valid (may be holes) in some
> cases. For example, if the pageblock order is MAX_ORDER - 1, 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>
> ---
>  mm/page_alloc.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6457b64fe562..dc4005b32ae0 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1502,6 +1502,14 @@ 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: if the start pfn of a pageblock is valid, but it can not always guarantee
> + * the end pfn of the pageblock is also valid (may be holes) in some cases. For

"valid" sounds confusing here.  pfn_valid() is true, but the pfn is
considered invalid at some degree.  How about the following?

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 - 1, 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 this possible issue when checking if the whole pfns are valid of a
                                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

whether all pfns of a pageblock are valid. ?

> + * pageblock.
>   */
>  struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
>  				     unsigned long end_pfn, struct zone *zone)

My English is poor.  So, feel free to ignore the comments.

Best Regards,
Huang, Ying