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)
