[PATCH 08/18] raid6: warn when using less than four devices

Quoting H. Peter Anvin who came up with the RAID6 P/Q algorithm, and
who wrote the initial implementation, then still part of the md driver:

  The RAID-6 code has *never* supported only 3 units, and if it ever
  worked for *any* of the implementations it was purely by accident.
  Speaking as the original author I should know; this was deliberate as
  in some cases the degenerate case (3) would have required extra trays
  in the code to no user benefit.

While md never allowed less than 4 devices, btrfs does.  This new
warning will trigger for such file systems, but given how it already
causes havoc that is a good thing.  If btrfs wants to fix third, it
should switch to transparently use three-way mirroring underneath,
which will work as P and Q are copies of the single data device by
the definition of the Linux RAID 6 P/Q algorithm.

Signed-off-by: Christoph Hellwig <[email protected]>
Acked-by: Ard Biesheuvel <[email protected]>
Tested-by: Ard Biesheuvel <[email protected]> # kunit only on arm64
---
 include/linux/raid/pq.h | 2 ++
 lib/raid/raid6/algos.c  | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/include/linux/raid/pq.h b/include/linux/raid/pq.h
index 425a227591c0..87e3cb55bf07 100644
--- a/include/linux/raid/pq.h
+++ b/include/linux/raid/pq.h
@@ -11,6 +11,8 @@
 #include <linux/blkdev.h>
 #include <linux/mm.h>
 
+#define RAID6_MIN_DISKS		4
+
 void raid6_gen_syndrome(int disks, size_t bytes, void **ptrs);
 void raid6_xor_syndrome(int disks, int start, int stop, size_t bytes,
 		void **ptrs);
diff --git a/lib/raid/raid6/algos.c b/lib/raid/raid6/algos.c
index b0ba31f6d48e..63d1945ba63c 100644
--- a/lib/raid/raid6/algos.c
+++ b/lib/raid/raid6/algos.c
@@ -42,6 +42,7 @@ void raid6_gen_syndrome(int disks, size_t bytes, void **ptrs)
 {
 	WARN_ON_ONCE(!in_task() || irqs_disabled() || softirq_count());
 	WARN_ON_ONCE(bytes & 511);
+	WARN_ON_ONCE(disks < RAID6_MIN_DISKS);
 
 	raid6_call.gen_syndrome(disks, bytes, ptrs);
 }
@@ -77,6 +78,7 @@ void raid6_xor_syndrome(int disks, int start, int stop, size_t bytes,
 {
 	WARN_ON_ONCE(!in_task() || irqs_disabled() || softirq_count());
 	WARN_ON_ONCE(bytes & 511);
+	WARN_ON_ONCE(disks < RAID6_MIN_DISKS);
 	WARN_ON_ONCE(stop < start);
 
 	raid6_call.xor_syndrome(disks, start, stop, bytes, ptrs);
-- 
2.53.0