The patch titled Subject: mm/mempolicy: fix a race between offset_il_node and mpol_rebind_task has been added to the -mm tree. Its filename is mm-mempolicy-fix-a-race-between-offset_il_node-and-mpol_rebind_task.patch

This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/mm-mempolicy-fix-a-race-between-of... and later at https://ozlabs.org/~akpm/mmotm/broken-out/mm-mempolicy-fix-a-race-between-of...

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------------------------------------------------------ From: yanghui yanghui.def@bytedance.com Subject: mm/mempolicy: fix a race between offset_il_node and mpol_rebind_task

Servers happened below panic: Kernel version:5.4.56 BUG: unable to handle page fault for address: 0000000000002c48 RIP: 0010:__next_zones_zonelist+0x1d/0x40 [264003.977696] RAX: 0000000000002c40 RBX: 0000000000100dca RCX: 0000000000000014 [264003.977872] Call Trace: [264003.977888] __alloc_pages_nodemask+0x277/0x310 [264003.977908] alloc_page_interleave+0x13/0x70 [264003.977926] handle_mm_fault+0xf99/0x1390 [264003.977951] __do_page_fault+0x288/0x500 [264003.977979] ? schedule+0x39/0xa0 [264003.977994] do_page_fault+0x30/0x110 [264003.978010] page_fault+0x3e/0x50

The reason for the panic is that MAX_NUMNODES is passed in the third parameter in __alloc_pages_nodemask(preferred_nid). So access to zonelist->zoneref->zone_idx in __next_zones_zonelist will cause a panic.

In offset_il_node(), first_node() returns nid from pol->v.nodes, after this other threads may chang pol->v.nodes before next_node(). This race condition will let next_node return MAX_NUMNODES. So put pol->nodes in a local variable.

The race condition is between offset_il_node and cpuset_change_task_nodemask: CPU0: CPU1: alloc_pages_vma() interleave_nid(pol,) offset_il_node(pol,) first_node(pol->v.nodes) cpuset_change_task_nodemask //nodes==0xc mpol_rebind_task mpol_rebind_policy mpol_rebind_nodemask(pol,nodes) //nodes==0x3 next_node(nid, pol->v.nodes)//return MAX_NUMNODES

Link: https://lkml.kernel.org/r/20210906034658.48721-1-yanghui.def@bytedance.com Signed-off-by: yanghui yanghui.def@bytedance.com Reviewed-by: Muchun Song songmuchun@bytedance.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/mempolicy.c | 17 +++++++++++++---- 1 file changed, 13 insertions(+), 4 deletions(-)

--- a/mm/mempolicy.c~mm-mempolicy-fix-a-race-between-offset_il_node-and-mpol_rebind_task +++ a/mm/mempolicy.c @@ -1979,17 +1979,26 @@ unsigned int mempolicy_slab_node(void) */ static unsigned offset_il_node(struct mempolicy *pol, unsigned long n) { - unsigned nnodes = nodes_weight(pol->nodes); - unsigned target; + nodemask_t nodemask = pol->nodes; + unsigned int target, nnodes; int i; int nid; + /* + * The barrier will stabilize the nodemask in a register or on + * the stack so that it will stop changing under the code. + * + * Between first_node() and next_node(), pol->nodes could be changed + * by other threads. So we put pol->nodes in a local stack. + */ + barrier();

+ nnodes = nodes_weight(nodemask); if (!nnodes) return numa_node_id(); target = (unsigned int)n % nnodes; - nid = first_node(pol->nodes); + nid = first_node(nodemask); for (i = 0; i < target; i++) - nid = next_node(nid, pol->nodes); + nid = next_node(nid, nodemask); return nid; }

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Patches currently in -mm which might be from yanghui.def@bytedance.com are

mm-mempolicy-fix-a-race-between-offset_il_node-and-mpol_rebind_task.patch