On 5.06.19 г. 12:14 ч., Qu Wenruo wrote:
>
>
> On 2019/6/3 下午6:06, Nikolay Borisov wrote:
>> Currently the first megabyte on a device housing a btrfs filesystem is
>> exempt from allocation and trimming. Currently this is not a problem
>> since 'start' is set to 1m at the beginning of btrfs_trim_free_extents
>> and find_first_clear_extent_bit always returns a range that is >= start.
>> However, in a follow up patch find_first_clear_extent_bit will be
>> changed such that it will return a range containing 'start' and this
>> range may very well be 0...>=1M so 'start'.
>>
>> Future proof the sole user of find_first_clear_extent_bit by setting
>> 'start' after the function is called. No functional changes.
>>
>> Signed-off-by: Nikolay Borisov <nbori...@suse.com>
>
> Doesn't that previous patch already address this by:
>
> + u64 start = SZ_1M, len = 0, end = 0;
No, because with the changes introduced in the next patch start can
actually be made to point to 0 for example. One of the self-test cases
covers this, e.g. :
find_first_clear_extent_bit(&tree, SZ_512K, &start, &end,
+ CHUNK_TRIMMED | CHUNK_ALLOCATED);
>
> Thanks,
> Qu
>
>
>> ---
>> fs/btrfs/extent-tree.c | 4 ++++
>> 1 file changed, 4 insertions(+)
>>
>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>> index d8c5febf7636..5a11e4988243 100644
>> --- a/fs/btrfs/extent-tree.c
>> +++ b/fs/btrfs/extent-tree.c
>> @@ -11183,6 +11183,10 @@ static int btrfs_trim_free_extents(struct
>> btrfs_device *device, u64 *trimmed)
>> * to the caller to trim the value to the size of the device.
>> */
>> end = min(end, device->total_bytes - 1);
>> +
>> + /* Ensure we skip first mb in case we have a bootloader there */
>> + start = max_t(u64, start, SZ_1M);
>> +
>> len = end - start + 1;
>>
>> /* We didn't find any extents */
>>
>
