Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set pageblock_skip on reserved pages

[Vlastimil Babka]

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