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

[Andrea Arcangeli <aarcange@redhat.com>]

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