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

[Mike Rapoport <rppt@linux.ibm.com>]

On Wed, Nov 25, 2020 at 07:45:30AM +0100, David Hildenbrand wrote:
> 
> > 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.

My guest would be that it is me and Baoquan:
73a6e474cb37 ("mm: memmap_init: iterate over memblock regions rather that check each PFN")

Until then reserved pages were traversed in memmap_init_zone() and after
the change they are not because on x86 reserved memory is not considered
memory for some reason.

Can you please check if this untested patch helps:

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index eaa227a479e4..be3c85a6714e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6191,7 +6191,9 @@ void __meminit __weak memmap_init(unsigned long size, int nid,
 {
 	unsigned long start_pfn, end_pfn;
 	unsigned long range_end_pfn = range_start_pfn + size;
+	phys_addr_t start, end;
 	int i;
+	u64 j;
 
 	for_each_mem_pfn_range(i, nid, &start_pfn, &end_pfn, NULL) {
 		start_pfn = clamp(start_pfn, range_start_pfn, range_end_pfn);
@@ -6203,6 +6205,19 @@ void __meminit __weak memmap_init(unsigned long size, int nid,
 					 MEMINIT_EARLY, NULL, MIGRATE_MOVABLE);
 		}
 	}
+
+	__for_each_mem_range(j, &memblock.reserved, NULL, nid,	MEMBLOCK_NONE,
+			     &start, &end, NULL) {
+		start_pfn = clamp(PHYS_PFN(start), range_start_pfn,
+				  range_end_pfn);
+		end_pfn = clamp(PHYS_PFN(end), range_start_pfn, range_end_pfn);
+
+		if (end_pfn > start_pfn) {
+			size = end_pfn - start_pfn;
+			memmap_init_zone(size, nid, zone, start_pfn,
+					 MEMINIT_EARLY, NULL, MIGRATE_MOVABLE);
+		}
+	}
 }
 
 static int zone_batchsize(struct zone *zone)

> > 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
> 

-- 
Sincerely yours,
Mike.