On Fri, Mar 17, 2017 at 01:31:23PM +0100, Alexander Potapenko wrote:

On Fri, Mar 17, 2017 at 1:08 PM, Peter Zijlstra [off-list ref] wrote:

quoted

On Thu, Mar 16, 2017 at 05:15:19PM -0700, Michael Davidson wrote:

quoted

Replace a variable length array in a struct by allocating
the memory for the entire struct in a char array on the stack.

Signed-off-by: Michael Davidson <redacted>
---
 drivers/md/raid10.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 063c43d83b72..158ebdff782c 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c

@@ -4654,11 +4654,10 @@ static int handle_reshape_read_error(struct mddev *mddev,
      /* Use sync reads to get the blocks from somewhere else */
      int sectors = r10_bio->sectors;
      struct r10conf *conf = mddev->private;
-     struct {
-             struct r10bio r10_bio;
-             struct r10dev devs[conf->copies];
-     } on_stack;
-     struct r10bio *r10b = &on_stack.r10_bio;
+     char on_stack_r10_bio[sizeof(struct r10bio) +
+                           conf->copies * sizeof(struct r10dev)]
+                           __aligned(__alignof__(struct r10bio));
+     struct r10bio *r10b = (struct r10bio *)on_stack_r10_bio;
      int slot = 0;
      int idx = 0;
      struct bio_vec *bvec = r10_bio->master_bio->bi_io_vec;

That's disgusting. Why not fix LLVM to support this?

IIUC there's only a handful of VLAIS instances in LLVM code, why not
just drop them for the sake of better code portability?
(To quote Linus, "this feature is an abomination":
https://lkml.org/lkml/2013/9/23/500)

Be that as it may; what you construct above is disgusting. Surely the
code can be refactored to not look like dog vomit?

Also; its not immediately obvious conf->copies is 'small' and this
doesn't blow up the stack; I feel that deserves a comment somewhere.