With some code that expands the zone boundaries, VM_BUG_ON(bad_range()) was being triggered.
Previously, page_outside_zone_boundaries() decided that once it detected
a page outside the boundaries, it was certainly outside even if the
seqlock indicated the data was invalid & needed to be reread. This
methodology _almost_ works because zones are only ever grown. However,
becase the zone span is stored as a start and a length, some expantions
momentarily appear as shifts to the left (when the zone_start_pfn is
assigned prior to zone_spanned_pages).
If we want to remove the seqlock around zone_start_pfn & zone
spanned_pages, always writing the spanned_pages first, issuing a memory
barrier, and then writing the new zone_start_pfn _may_ work. The concern
there is that we could be seen as shrinking the span when zone_start_pfn
is written (the entire span would shift to the left). As there will be
no pages in the exsess span that actually belong to the zone being
manipulated, I don't expect there to be issues.
Signed-off-by: Cody P Schafer <cody@xxxxxxxxxxxxxxxxxx>
---
mm/page_alloc.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 97bdf6b..a54baa9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -238,12 +238,13 @@ bool oom_killer_disabled __read_mostly;
#ifdef CONFIG_DEBUG_VM
static int page_outside_zone_boundaries(struct zone *zone, struct page *page)
{
- int ret = 0;
+ int ret;
unsigned seq;
unsigned long pfn = page_to_pfn(page);
unsigned long sp, start_pfn;
do {
+ ret = 0;
seq = zone_span_seqbegin(zone);
start_pfn = zone->zone_start_pfn;
sp = zone->spanned_pages;
--
1.8.2.1
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