Hello, On Mon, Nov 23, 2020 at 02:01:16PM +0100, Vlastimil Babka wrote: > On 11/21/20 8:45 PM, Andrea Arcangeli wrote: > > A corollary issue was fixed in > > 39639000-39814fff : Unknown E820 type > > > > pfn 0x7a200 -> 0x7a200000 min_pfn hit non-RAM: > > > > 7a17b000-7a216fff : Unknown E820 type > > It would be nice to also provide a /proc/zoneinfo and how exactly the > "zone_spans_pfn" was violated. I assume we end up below zone's > start_pfn, but is it true? Agreed, I was about to grab that info along with all page struct around the pfn 0x7a200 and phys address 0x7a216fff. # grep -A1 E820 /proc/iomem 7a17b000-7a216fff : Unknown E820 type 7a217000-7bffffff : System RAM DMA zone_start_pfn 1 zone_end_pfn() 4096 contiguous 1 DMA32 zone_start_pfn 4096 zone_end_pfn() 1048576 contiguous 0 Normal zone_start_pfn 1048576 zone_end_pfn() 4715392 contiguous 1 Movable zone_start_pfn 0 zone_end_pfn() 0 contiguous 0 500222 0x7a1fe000 0x1fff000000001000 reserved True 500223 0x7a1ff000 0x1fff000000001000 reserved True # I suspect "highest pfn" was somewhere in the RAM range # 0x7a217000-0x7a400000 and the pageblock_start_pfn(pfn) # made highest point to pfn 0x7a200 physaddr 0x7a200000 # below, which is reserved indeed since it's non-RAM # first number is pfn hex(500224) == 0x7a200 pfn physaddr page->flags 500224 0x7a200000 0x1fff000000001000 reserved True 500225 0x7a201000 0x1fff000000001000 reserved True *snip* 500245 0x7a215000 0x1fff000000001000 reserved True 500246 0x7a216000 0x1fff000000001000 reserved True 500247 0x7a217000 0x3fff000000000000 reserved False 500248 0x7a218000 0x3fff000000000000 reserved False All RAM pages non-reserved are starting at 0x7a217000 as expected. The non-RAM page_zonenum(pfn_to_page(0x7a200)) points to ZONE_DMA and page_zone(page) below was the DMA zone despite the pfn of 0x7a200 is in DMA32. VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page); So the patch I sent earlier should prevent the above BUG_ON by not setting highest to 0x7a200 when pfn is in the phys RAM range 0x7a217000-0x7a400000, because pageblock_pfn_to_page will notice that the zone is the wrong one. if (page_zone(start_page) != zone) return NULL; However the real bug seems that reserved pages have a zero zone_id in the page->flags when it should have the real zone id/nid. The patch I sent earlier to validate highest would only be needed to deal with pfn_valid. Something must have changed more recently than v5.1 that caused the zoneid of reserved pages to be wrong, a possible candidate for the real would be this change below: + __init_single_page(pfn_to_page(pfn), pfn, 0, 0); Even if it may not be it, at the light of how the reserved page zoneid/nid initialized went wrong, the above line like it's too flakey to stay. It'd be preferable if the pfn_valid fails and the pfn_to_section_nr(pfn) returns an invalid section for the intermediate step. Even better memset 0xff over the whole page struct until the second stage comes around. Whenever pfn_valid is true, it's better that the zoneid/nid is correct all times, otherwise if the second stage fails we end up in a bug with weird side effects. Maybe it's not the above that left a zero zoneid though, I haven't tried to bisect it yet to look how the page->flags looked like on a older kernel that didn't seem to reproduce this crash, I'm just guessing. Thanks, Andrea
