Re: [Qemu-devel] [PATCH 5/5] block: Avoid bdrv_get_geometry() where errors should be detected

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> Am 09.05.2014 um 11:48 hat Markus Armbruster geschrieben:
>> bdrv_get_geometry() hides errors.  Use bdrv_nb_sectors() or
>> bdrv_getlength() instead where that's obviously inappropriate.
>> 
>> Signed-off-by: Markus Armbruster <address@hidden>
>
>> @@ -1167,7 +1174,7 @@ static int img_convert(int argc, char **argv)
>>      BlockDriver *drv, *proto_drv;
>>      BlockDriverState **bs = NULL, *out_bs = NULL;
>>      int64_t total_sectors, nb_sectors, sector_num, bs_offset;
>> -    uint64_t bs_sectors;
>> +    int64_t *bs_sectors = NULL;
>>      uint8_t * buf = NULL;
>>      size_t bufsectors = IO_BUF_SIZE / BDRV_SECTOR_SIZE;
>>      const uint8_t *buf1;
>> @@ -1307,7 +1314,8 @@ static int img_convert(int argc, char **argv)
>>  
>>      qemu_progress_print(0, 100);
>>  
>> -    bs = g_malloc0(bs_n * sizeof(BlockDriverState *));
>> +    bs = g_new(BlockDriverState *, bs_n);
>> +    bs_sectors = g_new(int64_t, bs_n);
>>  
>>      total_sectors = 0;
>>      for (bs_i = 0; bs_i < bs_n; bs_i++) {
>> @@ -1321,8 +1329,14 @@ static int img_convert(int argc, char **argv)
>>              ret = -1;
>>              goto out;
>>          }
>> -        bdrv_get_geometry(bs[bs_i], &bs_sectors);
>> -        total_sectors += bs_sectors;
>> +        bs_sectors[bs_i] = bdrv_nb_sectors(bs[bs_i]);
>> +        if (bs_sectors[bs_i] < 0) {
>> +            error_report("Could not get size of %s: %s",
>> +                         argv[optind + bs_i], strerror(-bs_sectors[bs_i]));
>> +            ret = -1;
>> +            goto out;
>> +        }
>> +        total_sectors += bs_sectors[bs_i];
>>      }
>>  
>>      if (sn_opts) {
>> @@ -1446,7 +1460,6 @@ static int img_convert(int argc, char **argv)
>>  
>>      bs_i = 0;
>>      bs_offset = 0;
>> -    bdrv_get_geometry(bs[0], &bs_sectors);
>>  
>>      /* increase bufsectors from the default 4096 (2M) if opt_transfer_length
>>       * or discard_alignment of the out_bs is greater. Limit to 32768 (16MB)
>> @@ -1512,19 +1525,19 @@ static int img_convert(int argc, char **argv)
>>              buf2 = buf;
>>              while (remainder > 0) {
>>                  int nlow;
>> -                while (bs_num == bs_sectors) {
>> +                while (bs_num == bs_sectors[bs_i]) {
>>                      bs_i++;
>>                      assert (bs_i < bs_n);
>> -                    bs_offset += bs_sectors;
>> -                    bdrv_get_geometry(bs[bs_i], &bs_sectors);
>> +                    bs_offset += bs_sectors[bs_i];
>
> bs_i is already incremented here, so you're accessing the new bs_sectors
> instead of the old one.

Before my patch:

1535            while (bs_num == bs_sectors) {
1536                bs_i++;
1537                assert (bs_i < bs_n);
1538                bs_offset += bs_sectors;
1539                bdrv_get_geometry(bs[bs_i], &bs_sectors);
1540                bs_num = 0;
1541                /* printf("changing part: sector_num=%" PRId64 ", "
1542                   "bs_i=%d, bs_offset=%" PRId64 ", bs_sectors=%" PRId64
1543                   "\n", sector_num, bs_i, bs_offset, bs_sectors); */
1544            }

Before the loop, bs_i is 0, and bs_sectors is the number of sectors in
bs[0] (see previous hunk).

In the i-th iteration (counting from 1):

    1538 use number of sectors in bs[i-1]
    1539 compute number of sectors in bs[i]
    1544 use number of sectors in bs[i]

After my patch:

1548            while (bs_num == bs_sectors[bs_i]) {
1549                bs_i++;
1550                assert (bs_i < bs_n);
1551                bs_offset += bs_sectors[bs_i];
1552                bs_num = 0;
1553                /* printf("changing part: sector_num=%" PRId64 ", "
1554                   "bs_i=%d, bs_offset=%" PRId64 ", bs_sectors=%" PRId64
1555                   "\n", sector_num, bs_i, bs_offset, bs_sectors[bs_i]); */
1556            }

You're right.  I'll fix it to look like the other loop, where I got the
order right.

>>                      bs_num = 0;
>>                      /* printf("changing part: sector_num=%" PRId64 ", "
>>                         "bs_i=%d, bs_offset=%" PRId64 ", bs_sectors=%" PRId64
>> -                       "\n", sector_num, bs_i, bs_offset, bs_sectors); */
>> +                       "\n", sector_num, bs_i, bs_offset, 
>> bs_sectors[bs_i]); */
>>                  }
>> -                assert (bs_num < bs_sectors);
>> +                assert (bs_num < bs_sectors[bs_i]);
>>  
>> -                nlow = (remainder > bs_sectors - bs_num) ? bs_sectors - 
>> bs_num : remainder;
>> +                nlow = remainder > bs_sectors[bs_i] - bs_num
>> +                    ? bs_sectors[bs_i] - bs_num : remainder;
>>  
>>                  ret = bdrv_read(bs[bs_i], bs_num, buf2, nlow);
>>                  if (ret < 0) {
>> @@ -1585,14 +1598,13 @@ restart:
>>                  break;
>>              }
>>  
>> -            while (sector_num - bs_offset >= bs_sectors) {
>> +            while (sector_num - bs_offset >= bs_sectors[bs_i]) {
>> +                bs_offset += bs_sectors[bs_i];
>>                  bs_i ++;
>
> Here you got the order right.
>
>>                  assert (bs_i < bs_n);
>> -                bs_offset += bs_sectors;
>> -                bdrv_get_geometry(bs[bs_i], &bs_sectors);
>>                  /* printf("changing part: sector_num=%" PRId64 ", bs_i=%d, "
>>                    "bs_offset=%" PRId64 ", bs_sectors=%" PRId64 "\n",
>> -                   sector_num, bs_i, bs_offset, bs_sectors); */
>> +                   sector_num, bs_i, bs_offset, bs_sectors[bs_i]); */
>>              }
>>  
>>              if ((out_baseimg || has_zero_init) &&
>> @@ -1645,7 +1657,7 @@ restart:
>>                  }
>>              }
>>  
>> -            n = MIN(n, bs_sectors - (sector_num - bs_offset));
>> +            n = MIN(n, bs_sectors[bs_i] - (sector_num - bs_offset));
>>  
>>              sectors_read += n;
>>              if (count_allocated_sectors) {
>> @@ -1703,6 +1715,7 @@ out:
>>          }
>>          g_free(bs);
>>      }
>> +    g_free(bs_sectors);
>>  fail_getopt:
>>      g_free(options);
>
> This is not a trivial change that can be done while you're touching the
> code for fixing the error paths. The error checking is only a small part
> of it, while the main objective seems to be caching image sizes so that
> bdrv_getlength() needs to be called only once. I'm not sure if this
> optimisation is worthwhile, but if so, it should be a separate patch.

The main objective is to get rid of bdrv_get_geometry() without having
to recheck errors again and again :)

But point taken.  I'll split this up.