[PATCH v2] mm/sparse.c: fix error path in sparse_add_one_section

[Ross Zwisler <ross.zwisler@linux.intel.com>]

The following commit in -next:

commit 054620849110 ("mm/sparse.c: make sparse_init_one_section void and
remove check")

changed how the error handling in sparse_add_one_section() works.

Previously sparse_index_init() could return -EEXIST, and the function would
continue on happily.  'ret' would get unconditionally overwritten by the
result from sparse_init_one_section() and the error code after the 'out:'
label wouldn't be triggered.

With the above referenced commit, though, an -EEXIST error return from
sparse_index_init() now takes us through the function and into the error
case after 'out:'.  This eventually causes a kernel BUG, probably because
we've just freed a memory section that we successfully set up and marked as
present:

  BUG: unable to handle kernel paging request at ffffea0005000080
  RIP: 0010:memmap_init_zone+0x154/0x1cf

  Call Trace:
   move_pfn_range_to_zone+0x168/0x180
   devm_memremap_pages+0x29b/0x480
   pmem_attach_disk+0x1ae/0x6c0 [nd_pmem]
   ? devm_memremap+0x79/0xb0
   nd_pmem_probe+0x7e/0xa0 [nd_pmem]
   nvdimm_bus_probe+0x6e/0x160 [libnvdimm]
   driver_probe_device+0x310/0x480
   __device_attach_driver+0x86/0x100
   ? __driver_attach+0x110/0x110
   bus_for_each_drv+0x6e/0xb0
   __device_attach+0xe2/0x160
   device_initial_probe+0x13/0x20
   bus_probe_device+0xa6/0xc0
   device_add+0x41b/0x660
   ? lock_acquire+0xa3/0x210
   nd_async_device_register+0x12/0x40 [libnvdimm]
   async_run_entry_fn+0x3e/0x170
   process_one_work+0x230/0x680
   worker_thread+0x3f/0x3b0
   kthread+0x12f/0x150
   ? process_one_work+0x680/0x680
   ? kthread_create_worker_on_cpu+0x70/0x70
   ret_from_fork+0x3a/0x50

Fix this by clearing 'ret' back to 0 if sparse_index_init() returns
-EEXIST.  This restores the previous behavior.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
---
 mm/sparse.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/sparse.c b/mm/sparse.c
index f55e79fda03e..eb188eb6b82d 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -770,6 +770,7 @@ int __meminit sparse_add_one_section(struct pglist_data *pgdat,
 	ret = sparse_index_init(section_nr, pgdat->node_id);
 	if (ret < 0 && ret != -EEXIST)
 		return ret;
+	ret = 0;
 	memmap = kmalloc_section_memmap(section_nr, pgdat->node_id, altmap);
 	if (!memmap)
 		return -ENOMEM;
-- 
2.14.4

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