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

[David Hildenbrand <david@redhat.com>]

> Am 25.11.2020 um 06:34 schrieb 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);
> 

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