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
