Re: [RFC v3 3/5] file-posix: introduce get_sysfs_long_val for zoned device information.

[Stefan Hajnoczi]

On Mon, Jun 27, 2022 at 08:19:15AM +0800, Sam Li wrote:
> Use sysfs attribute files to get the zoned device information in case
> that ioctl() commands of zone management interface won't work. It can
> return long type of value like chunk_sectors, zoned_append_max_bytes,
> max_open_zones, max_active_zones.
> ---
>  block/file-posix.c | 37 +++++++++++++++++++++++++------------
>  1 file changed, 25 insertions(+), 12 deletions(-)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index 1b8b0d351f..73c2cdfbca 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -1216,15 +1216,19 @@ static int hdev_get_max_hw_transfer(int fd, struct 
> stat *st)
>  #endif
>  }
>  
> -static int hdev_get_max_segments(int fd, struct stat *st)
> -{
> +/*
> + * Get zoned device information (chunk_sectors, zoned_append_max_bytes,
> + * max_open_zones, max_active_zones) through sysfs attribute files.

This function is also used to get max_segments, which is not related to
zoned devices. How about:

  Get a block queue sysfs attribute value.

> + */
> +static long get_sysfs_long_val(int fd, struct stat *st,
> +                               const char *attribute) {
>  #ifdef CONFIG_LINUX
>      char buf[32];
>      const char *end;
>      char *sysfspath = NULL;
>      int ret;
>      int sysfd = -1;
> -    long max_segments;
> +    long val;
>  
>      if (S_ISCHR(st->st_mode)) {
>          if (ioctl(fd, SG_GET_SG_TABLESIZE, &ret) == 0) {
> @@ -1237,8 +1241,9 @@ static int hdev_get_max_segments(int fd, struct stat 
> *st)
>          return -ENOTSUP;
>      }
>  
> -    sysfspath = g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments",
> -                                major(st->st_rdev), minor(st->st_rdev));
> +    sysfspath = g_strdup_printf("/sys/dev/block/%u:%u/queue/%s",
> +                                major(st->st_rdev), minor(st->st_rdev),
> +                                attribute);
>      sysfd = open(sysfspath, O_RDONLY);
>      if (sysfd == -1) {
>          ret = -errno;
> @@ -1256,9 +1261,9 @@ static int hdev_get_max_segments(int fd, struct stat 
> *st)
>      }
>      buf[ret] = 0;
>      /* The file is ended with '\n', pass 'end' to accept that. */
> -    ret = qemu_strtol(buf, &end, 10, &max_segments);
> +    ret = qemu_strtol(buf, &end, 10, &val);
>      if (ret == 0 && end && *end == '\n') {
> -        ret = max_segments;
> +        ret = val;
>      }
>  
>  out:
> @@ -1272,6 +1277,15 @@ out:
>  #endif
>  }
>  
> +static int hdev_get_max_segments(int fd, struct stat *st) {
> +    int ret;
> +    ret = get_sysfs_long_val(fd, st, "max_segments");
> +    if (ret < 0) {
> +        return -1;

This hides the actual error number. The if statement can be dropped and
the function can be simplified to:

  static int hdev_get_max_segments(int fd, struct stat *st) {
      return get_sysfs_long_val(fd, st, "max_segments");
  }

Whether you want to keep the tiny helper function or inline
get_sysfs_long_val(fd, st, "max_segments") into the caller is up to you.

> +    }
> +    return ret;
> +}
> +
>  static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
>  {
>      BDRVRawState *s = bs->opaque;
> @@ -1872,6 +1886,7 @@ static int handle_aiocb_zone_report(void *opaque) {
>  
>  static int handle_aiocb_zone_mgmt(void *opaque) {
>      RawPosixAIOData *aiocb = opaque;
> +    BlockDriverState *s = aiocb->bs;
>      int fd = aiocb->aio_fildes;
>      int64_t offset = aiocb->aio_offset;
>      int64_t len = aiocb->aio_nbytes;
> @@ -1884,11 +1899,9 @@ static int handle_aiocb_zone_mgmt(void *opaque) {
>      int64_t zone_size_mask;
>      int ret;
>  
> -    ret = ioctl(fd, BLKGETZONESZ, &zone_size);
> -    if (ret) {
> -        return -1;
> -    }
> -
> +    g_autofree struct stat *file = g_new(struct stat, 1);
> +    stat(s->filename, file);

There is no need to allocate struct stat using g_new(). It's a small
struct that can be on the stack.

Also, fstat(fd, &st) is preferred when the file descriptor is already
open because it avoids race conditions due to file renaming/path
traversal.

Here is how this could be written:

  struct stat st;
  if (fstat(fd, &st) < 0) {
      return -errno;
  }

> +    zone_size = get_sysfs_long_val(fd, file, "chunk_sectors");
>      zone_size_mask = zone_size - 1;
>      if (offset & zone_size_mask) {
>          error_report("offset is not the start of a zone");
> -- 
> 2.35.3
>

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