Re: [PATCH 17/32] btrfs: extent_io: don't allow tree block to cross page boundary for subpage support

[Nikolay Borisov <nborisov@suse.com>]

On 6.11.20 г. 13:54 ч., Nikolay Borisov wrote:
> 
> 
> On 3.11.20 г. 15:30 ч., Qu Wenruo wrote:
>> As a preparation for subpage sector size support (allowing filesystem
>> with sector size smaller than page size to be mounted) if the sector
>> size is smaller than page size, we don't allow tree block to be read if
>> it crosses 64K(*) boundary.
>>
>> The 64K is selected because:
>> - We are only going to support 64K page size for subpage for now
>> - 64K is also the max node size btrfs supports
>>
>> This ensures that, tree blocks are always contained in one page for a
>> system with 64K page size, which can greatly simplify the handling.
>>
>> Or we need to do complex multi-page handling for tree blocks.
>>
>> Currently the only way to create such tree blocks crossing 64K boundary
>> is by btrfs-convert, which will get fixed soon and doesn't get
>> wide-spread usage.
> 
> So filesystems with subpage blocksize which have been created as a
> result of a convert operation would eventually fail to read some block
> am I correct in my understanding? If that is the case then can't we
> simply land subpage support in userspace tools _after_ the convert has
> been fixed and turn this check into an assert?
> 
> 
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>>  fs/btrfs/extent_io.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
>> index 30768e49cf47..30bbaeaa129a 100644
>> --- a/fs/btrfs/extent_io.c
>> +++ b/fs/btrfs/extent_io.c
>> @@ -5261,6 +5261,13 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
>>  		btrfs_err(fs_info, "bad tree block start %llu", start);
>>  		return ERR_PTR(-EINVAL);
>>  	}
>> +	if (btrfs_is_subpage(fs_info) && round_down(start, PAGE_SIZE) !=
>> +	    round_down(start + len - 1, PAGE_SIZE)) {

One more thing, instead of doing those 2 round_downs, why not:

offset_in_page(start) + len > PAGE_SIZE

>> +		btrfs_err(fs_info,
>> +		"tree block crosses page boundary, start %llu nodesize %lu",
>> +			  start, len);
>> +		return ERR_PTR(-EINVAL);
>> +	}
>>  
>>  	eb = find_extent_buffer(fs_info, start);
>>  	if (eb)
>>
>