Messages in this thread | | | Date | Tue, 25 Apr 2023 09:27:23 +0800 | Subject | Re: [PATCH v3 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page() | From | Baolin Wang <> |
| |
On 4/25/2023 8:22 AM, Huang, Ying wrote: > Baolin Wang <baolin.wang@linux.alibaba.com> 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@yhuang6-desk2.ccr.corp.intel.com/ >> >> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com> >> --- >> 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"?
| |