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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/28/26 22:19, Gregory Price (Meta) wrote: > For a bind or preferred-many policy, mempolicy_slab_node() tests the > first matching slot's zone for NULL, then reads the slot again to look > up the zone's node. Offlining a zone's last block rebuilds the > zonelists in place, so the second read can return the terminator: > > CPU 0 CPU 1 > mempolicy_slab_node() > zonelist_zone(slot 4) > = node 2 NORMAL > offline_pages() > build_all_zonelists() > slot 4 = NULL > zonelist_node_idx(slot 4) > zone_to_nid(NULL) > > Current GCC merges the two reads, but nothing requires it. With > mm/mempolicy.o built with -fno-inline -fno-ipa-sra, holding a bound > task's slab_strict_numa allocation between the reads while a movable > block earlier in the zonelist goes offline oopses every time: > > BUG: kernel NULL pointer dereference, address: 0000000000000060 > RIP: 0010:zone_to_nid+0x0/0x10 > Call Trace: > kmem_cache_alloc_noprof+0x294/0x610 > do_getname+0x34/0x230 > > mempolicy_slab_node() and zonelist_node_idx() are tail calls here, so > neither appears in the trace. > > Read the slot once and use that zone. > > Fixes: 11cd8638c37f ("mm, page_alloc: remove stop_machine from build_all_zonelists") > Cc: stable@vger.kernel.org > Signed-off-by: Gregory Price (Meta) Reviewed-by: Vlastimil Babka (SUSE) > --- > mm/mempolicy.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > index 0884c629670d..cd63c3e2c029 100644 > --- a/mm/mempolicy.c > +++ b/mm/mempolicy.c > @@ -2176,6 +2176,7 @@ unsigned int mempolicy_slab_node(void) > case MPOL_PREFERRED_MANY: > { > struct zoneref *z; > + struct zone *zone; > > /* > * Follow bind policy behavior and start allocation at the > @@ -2186,7 +2187,8 @@ unsigned int mempolicy_slab_node(void) > zonelist = &NODE_DATA(node)->node_zonelists[ZONELIST_FALLBACK]; > z = first_zones_zonelist(zonelist, highest_zoneidx, > &policy->nodes); > - return zonelist_zone(z) ? zonelist_node_idx(z) : node; > + zone = zonelist_zone(z); > + return zone ? zone_to_nid(zone) : node; > } > case MPOL_LOCAL: > return node;