block: fix oops on !disk->queue and sysfs discard alignment display

Eric Dumazet reports:

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At boot, I have a crash in part_discard_alignment_show+0x1b/0x50

CR2 : 000006ac

fault in : mov    0x2c(%rcx),%edx

I suspect commit 23ceb5b7719e9276d4 (block: Remove extra
discard_alignment from hd_struct) being in fault

----

Not quite known how ->queue can be NULL while the sysfs entry
exists, but lets play it safe and check for a NULL queue.
The rest of the sysfs show strategies in check.c do not dereference
disk->queue.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/fs/partitions/check.c b/fs/partitions/check.c
index 8ed4d3433199..f82e762eeca2 100644
--- a/fs/partitions/check.c
+++ b/fs/partitions/check.c
@@ -256,10 +256,12 @@ ssize_t part_discard_alignment_show(struct device *dev,
 {
         struct hd_struct *p = dev_to_part(dev);
         struct gendisk *disk = dev_to_disk(dev);
+        unsigned int alignment = 0;
 
-        return sprintf(buf, "%u\n",
-                        queue_limit_discard_alignment(&disk->queue->limits,
-                                                        p->start_sect));
+        if (disk->queue)
+                alignment = queue_limit_discard_alignment(&disk->queue->limits,
+                                                                p->start_sect);
+        return sprintf(buf, "%u\n", alignment);
 }
 
 ssize_t part_stat_show(struct device *dev,