Messages in this thread | | | From | Vlastimil Babka <> | Subject | Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set pageblock_skip on reserved pages | Date | Wed, 25 Nov 2020 13:08:54 +0100 |
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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.
In any case, compaction code can't fix this with better range checks.
> 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 >
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