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

[Mike Rapoport]

On Wed, Nov 25, 2020 at 02:32:02PM +0100, David Hildenbrand wrote:
> On 25.11.20 13:08, Vlastimil Babka wrote:
> > 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.
> 
> As we don't punch out pages of the memmap on x86-64, pfn_valid() keeps
> working as expected. There is a memmap that can be accessed and that was
> initialized.

I suspect that memmap for the reserved pages is not properly initialized
after recent changes in free_area_init(). They are cleared at
init_unavailable_mem() to have zone=0 and node=0, but they seem to be
never re-initialized with proper zone and node links which was not the
case before commit 73a6e474cb37 ("mm: memmap_init: iterate over memblock
regions rather that check each PFN").

Back then, memmap_init_zone() looped from zone_start_pfn till
zone_end_pfn and struct page for reserved pages with pfns inside the
zone would be initialized.

Now the loop is for interesection of [zone_start_pfn, zone_end_pfn] with
memblock.memory and for x86 reserved ranges are not in memblock.memory,
so the memmap for them remains semi-initialized.

> It's really just a matter of how to handle memory holes in
> this scenario.
> 
> a) Try initializing them to the covering node/zone (I gave one example
>    that might be tricky with hotplug)
> b) Mark such pages (either special node/zone or pagetype) and make pfn
>    walkers ignore these holes. For now, this can only be done via the
>    reserved flag.
> 
> -- 
> Thanks,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.