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

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Date:   Thu, 22 Jun 2017 11:49:56 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Tahsin Erdogan <tahsin@...gle.com>
Cc:     Andreas Dilger <adilger@...ger.ca>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 29/32] ext4: reserve space for xattr entries/names

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@...gle.com>

Thanks, applied.

					- Ted