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SubjectRe: [PATCH] md: raid10: remove VLAIS
On Thu, Oct 05 2017, Matthias Kaehlcke wrote:

> The raid10 driver can't be built with clang since it uses a variable
> length array in a structure (VLAIS):
>
> drivers/md/raid10.c:4583:17: error: fields must have a constant size:
> 'variable length array in structure' extension will never be supported
>
> Allocate the r10bio struct with kmalloc instead of using the VLAIS
> construct.
>
> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> ---
> drivers/md/raid10.c | 13 ++++++++-----
> 1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 374df5796649..9616163eaf8c 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -4578,15 +4578,16 @@ 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;
> + struct r10bio *r10b;
> int slot = 0;
> int idx = 0;
> struct page **pages;
>
> + r10b = kmalloc(sizeof(*r10b) +
> + sizeof(struct r10dev) * conf->copies, GFP_KERNEL);

GFP_KERNEL isn't a good idea here.
This could wait for writeback, and if writeback tries to write to the
region of the array which is being reshaped, it might deadlock.

GFP_NOIO is safer.

given that conf->copies is almost always 2 it might be nicer to
have

struct {
struct r10bio r10_bio;
struct r10dev devs[2];
} on_stack;

struct r10bio *r10b;

if (conf->copies <= ARRAY_SIZE(on_stack.devs))
r10b = &on_stack.r10_bio;
else
r10b = kmalloc(sizeof(*r10b) +
sizeof(struct r10dev) * conf->copies, GFP_NOIO);


Thanks,
NeilBrown


> + if (!r10b)
> + return -ENOMEM;
> +
> /* reshape IOs share pages from .devs[0].bio */
> pages = get_resync_pages(r10_bio->devs[0].bio)->pages;
>
> @@ -4635,11 +4636,13 @@ static int handle_reshape_read_error(struct mddev *mddev,
> /* couldn't read this block, must give up */
> set_bit(MD_RECOVERY_INTR,
> &mddev->recovery);
> + kfree(r10b);
> return -EIO;
> }
> sectors -= s;
> idx++;
> }
> + kfree(r10b);
> return 0;
> }
>
> --
> 2.14.2.920.gcf0c67979c-goog
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