> Am 25.11.2020 um 06:34 schrieb Andrea Arcangeli <aarcange@xxxxxxxxxx>: > > 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); > Before that change, the memmap of memory holes were only zeroed out. So the zones/nid was 0, however, pages were not reserved and had a refcount of zero - resulting in other issues. Most pfn walkers shouldn‘t mess with reserved pages and simply skip them. That would be the right fix here. > 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. I recently discussed with Baoquan to 1. Using a new pagetype to mark memory holes 2. Using special nid/zid to mark memory holes Setting the memmap to 0xff would be even more dangerous - pfn_zone() might simply BUG_ON. > > 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. Memory holes with a valid memmap might not have a zone/nid. For now, skipping reserved pages should be good enough, no? > > 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
