Re: [PATCH] md: raid10: remove VLAIS

[NeilBrown]

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 <m...@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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