Re: [PATCH v4 1/7] file-posix: fix max_iov for /dev/sg devices

[Eric Blake <eblake@redhat.com>]

On Tue, Jun 08, 2021 at 03:16:28PM +0200, Paolo Bonzini wrote:
> Even though it was only called for devices that have bs->sg set (which
> must be character devices), sg_get_max_segments looked at /sys/dev/block
> which only works for block devices.
> 
> On Linux the sg driver has its own way to provide the maximum number of
> iovecs in a scatter/gather list, so add support for it.  The block device
> path is kept because it will be reinstated in the next patches.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  block/file-posix.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index f37dfc10b3..536998a1d6 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -1180,6 +1180,17 @@ static int sg_get_max_segments(int fd)
>          goto out;
>      }
>  
> +    if (S_ISCHR(st.st_mode)) {
> +        if (ioctl(fd, SG_GET_SG_TABLESIZE, &ret) == 0) {

Do we need to do any conditional compilation based on whether
SG_GET_SG_TABLESIZE is a known ioctl, or is it old enough to be
assumed present on all platforms we care about?

> +            return ret;
> +        }
> +        return -ENOTSUP;
> +    }
> +
> +    if (!S_ISBLK(st.st_mode)) {
> +        return -ENOTSUP;
> +    }
> +
>      sysfspath = g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments",
>                                  major(st.st_rdev), minor(st.st_rdev));
>      sysfd = open(sysfspath, O_RDONLY);

Otherwise looks good to me.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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