Patch "md/raid5: limit request size according to implementation limits" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

md/raid5: limit request size according to implementation limits

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
md-raid5-limit-request-size-according-to-implementation-limits.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Konstantin Khlebnikov <[email protected]>
Date: Sun, 27 Nov 2016 19:32:32 +0300
Subject: md/raid5: limit request size according to implementation limits

From: Konstantin Khlebnikov <[email protected]>

commit e8d7c33232e5fdfa761c3416539bc5b4acd12db5 upstream.

Current implementation employ 16bit counter of active stripes in lower bits of bio->bi_phys_segments. If request is big enough to overflow this counter bio will be completed and freed too early.

Fortunately this not happens in default configuration because several other limits prevent that: stripe_cache_size * nr_disks effectively limits count of active stripes. And small max_sectors_kb at lower disks prevent that during normal read/write operations.

Overflow easily happens in discard if it's enabled by module parameter "devices_handle_discard_safely" and stripe_cache_size is set big enough.

This patch limits requests size with 256Mb - 8Kb to prevent overflows.

---
 drivers/md/raid5.c |    9 +++++++++
 1 file changed, 9 insertions(+)

@@ -6984,6 +6984,15 @@ static int raid5_run(struct mddev *mddev
 			stripe = (stripe | (stripe-1)) + 1;
 		mddev->queue->limits.discard_alignment = stripe;
 		mddev->queue->limits.discard_granularity = stripe;
+
+		/*
+		 * We use 16-bit counter of active stripes in bi_phys_segments
+		 * (minus one for over-loaded initialization)
+		 */
+		blk_queue_max_hw_sectors(mddev->queue, 0xfffe * STRIPE_SECTORS);
+		blk_queue_max_discard_sectors(mddev->queue,
+					      0xfffe * STRIPE_SECTORS);
+
 		/*
 		 * unaligned part of discard request will be ignored, so can't
 		 * guarantee discard_zeroes_data

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/md-raid5-limit-request-size-according-to-implementation-limits.patch