Re: size limit of extended attributes

[Jan Kara <jack@suse.cz>]

On Tue 05-05-15 15:38:11, Björn JACKE wrote:
> On 2015-05-02 at 01:43 +1000 Dave Chinner sent off:
> > On Thu, Apr 30, 2015 at 03:57:37PM +0200, Björn JACKE wrote:
> > > Can we get rid of the 64k size limit for EAs? The API on AIX is the same as on
> > > Linux.  But there is a huge size limit - which would help us already a lot.
> > 
> > No - the maximum xattr size of 64k is encoded into the on-disk
> > format of many filesystems and that's not a simple thing to change.
> 
> I know ext4 even has a much lower limit. But some filesystems don't have a
> limit, and there the kernel 64k limit strikes in. The EA size limit of some
> file systems could also be increased. As there is a real use case for that I
> guess filesystems would like to be able to support larger EA sizes.
  Yeah, so XFS could support more than 64K in principle if I look correct.
Supporting it for ext4 would mean ondisk format change - doable but
requires some non-trivial effort.

Regarding the API, the issue with larger xattr size is that currently we
copy whole xattr into kernel memory and process it in one go. Currently
that's OK but if you want xattrs that have megabytes, it may become an
effective way to DOS a system. So to support that we'd need to change at
least the API from VFS into filesystems so that xattrs could be processed
in smaller chunks. Again doable but quite some work.

All in all I don't think this is going to happen unless someone interested
in this invests significant amount of time to make this happen.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR
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