On 25.11.20 13:08, Vlastimil Babka wrote: > On 11/25/20 6:34 AM, Andrea Arcangeli wrote: >> 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 > > So the above means that around the "Unknown E820 type" we have: > > pfn 499712 - start of pageblock in ZONE_DMA32 > pfn 500091 - start of the "Unknown E820 type" range > pfn 500224 - start of another pageblock > pfn 500246 - end of "Unknown E820 type" > > So this is indeed not a zone boundary issue, but basically a hole not > aligned to pageblock boundary and really unexpected. > We have CONFIG_HOLES_IN_ZONE (that x86 doesn't set) for architectures > that do this, and even that config only affects pfn_valid_within(). But > here pfn_valid() is true, but the zone/node linkage is unexpected. > >> 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. > > Yeah I guess it would be simpler if zoneid/nid was correct for > pfn_valid() pfns within a zone's range, even if they are reserved due > not not being really usable memory. > > I don't think we want to introduce CONFIG_HOLES_IN_ZONE to x86. If the > chosen solution is to make this to a real hole, the hole should be > extended to MAX_ORDER_NR_PAGES aligned boundaries. As we don't punch out pages of the memmap on x86-64, pfn_valid() keeps working as expected. There is a memmap that can be accessed and that was initialized. It's really just a matter of how to handle memory holes in this scenario. a) Try initializing them to the covering node/zone (I gave one example that might be tricky with hotplug) b) Mark such pages (either special node/zone or pagetype) and make pfn walkers ignore these holes. For now, this can only be done via the reserved flag. -- Thanks, David / dhildenb
