Re: [PATCH v3 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.
>
> See below memory layout as an example and suppose the pageblock order
> is MAX_ORDER.
>
> [    0.000000] Zone ranges:
> [    0.000000]   DMA      [mem 0x0000000040000000-0x00000000ffffffff]
> [    0.000000]   DMA32    empty
> [    0.000000]   Normal   [mem 0x0000000100000000-0x0000001fa7ffffff]
> [    0.000000] Movable zone start for each node
> [    0.000000] Early memory node ranges
> [    0.000000]   node   0: [mem 0x0000000040000000-0x0000001fa3c7ffff]
> [    0.000000]   node   0: [mem 0x0000001fa3c80000-0x0000001fa3ffffff]
> [    0.000000]   node   0: [mem 0x0000001fa4000000-0x0000001fa402ffff]
> [    0.000000]   node   0: [mem 0x0000001fa4030000-0x0000001fa40effff]
> [    0.000000]   node   0: [mem 0x0000001fa40f0000-0x0000001fa73cffff]
> [    0.000000]   node   0: [mem 0x0000001fa73d0000-0x0000001fa745ffff]
> [    0.000000]   node   0: [mem 0x0000001fa7460000-0x0000001fa746ffff]
> [    0.000000]   node   0: [mem 0x0000001fa7470000-0x0000001fa758ffff]
> [    0.000000]   node   0: [mem 0x0000001fa7590000-0x0000001fa7dfffff]
>
> Focus on the last memory range, and there is a hole for the range [mem
> 0x0000001fa7590000-0x0000001fa7dfffff]. That means the last pageblock
> will contain the range from 0x1fa7c00000 to 0x1fa7ffffff, since the
> pageblock must be 4M aligned. And in this pageblock, these pfns will
> fall into 2 sub-section (the sub-section size is 2M aligned).
>
> So, the 1st sub-section (indicates pfn range: 0x1fa7c00000 -
> 0x1fa7dfffff ) in this pageblock is valid by calling subsection_map_init()
> in free_area_init(), but the 2nd sub-section (indicates pfn range:
> 0x1fa7e00000 - 0x1fa7ffffff ) in this pageblock is not 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>
> ---
> Changes from v2:
>  - Update the commit log and comments per Michal, thanks.
> Changes from v1:
>  - Update the comments per Ying and Mike, thanks.
>
> Note, I did not add Huang Ying's reviewed tag, since there are some
> updates per Michal's suggestion. Ying, please review the v3. Thanks.
> ---
>  mm/page_alloc.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6457b64fe562..bd124390c79b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1502,6 +1502,15 @@ 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 struct page even for a page block
> + * which contains a memory hole (i.e. there is no physical memory for a subset
> + * of the pfn range). 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 should be safe most of
> + * the time because struct pages are still zero pre-filled and pfn walkers

I don't think the pfn is just zero-filled even it's a hole.  Can you
confirm that?  In memmap_init() and memmap_init_zone_range(),
init_unavailable_range() is called to initialize the struct page.

Best Regards,
Huang, Ying

> + * shouldn't touch any physical memory range for which they do not recognize
> + * any specific metadata in struct pages.
>   */
>  struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
>  				     unsigned long end_pfn, struct zone *zone)