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:

> On 4/25/2023 8:22 AM, Huang, Ying wrote:
>> 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.
>
> Yes, what I mean is the page frames were initialized to zero firstly,
> and some fields were initialized to default value. The "zero
> pre-filled" seems confusing, may be change to "initialized"?

Yes.  That sounds good.

Best Regards,
Huang, Ying