Re: [PATCH 29/32] ext4: reserve space for xattr entries/names

["Theodore Ts'o" <tytso@mit.edu>]

On Wed, Jun 21, 2017 at 06:49:36PM -0700, Tahsin Erdogan wrote:
> New ea_inode feature allows putting large xattr values into external
> inodes. struct ext4_xattr_entry and the attribute name however have to
> remain in the inode extra space or external attribute block. Once that
> space is exhausted, no further entries can be added. Some of that space
> could also be used by values that fit in there at the time of addition.
> 
> So, a single xattr entry whose value barely fits in the external block
> could prevent further entries being added.
> 
> To mitigate the problem, this patch introduces a notion of reserve in
> the
> external attribute block that cannot be used by value data. This reserve
> is enforced when ea_inode feature is enabled. The amount of reserve is
> arbitrarily chosen to be min(block_size/8, 1024). The table below shows
> how much space is reserved for each block size and the guaranteed
> mininum
> number of entries that can be placed in the external attribute block.
> 
> block size     reserved bytes  entries (name length = 16)
>  1k            128              3
>  2k            256              7
>  4k            512             15
>  8k            1024            31
> 16k            1024            31
> 32k            1024            31
> 64k            1024            31
> 
> Signed-off-by: Tahsin Erdogan <tahsin@google.com>

Thanks, applied.

					- Ted