[PATCH 02/18] xstat: Add a pair of system calls to make extended file stats available [ver #6]

David Howells dhowells at redhat.com
Thu Jul 15 15:53:27 MDT 2010

Arnd Bergmann <arnd at arndb.de> wrote:

> > This was initially proposed as a set of xattrs, but the general preferance
> > is for an extended stat structure.
> I don't think I'd call this general preference. Three of the four
> are fixed length and could easily be done inside the structure if you
> leave a bit of space instead of a variable-length field at the end.


Maybe I wasn't clear: I meant having an extended stat() syscall rather than
using a bunch of getxattr()'s was the general preference.

> For the volume id, I could not find any file system that requires more
> than 32 bytes here, which is also reasonable to put into the structure.
> Make it 36 if you want to cover ascii encoded UUIDs.

You should also include a length.  Volume IDs may be binary rather than
NUL-terminated strings.

> That's at most 60 bytes for the extensions you're considering already,
> plus the 152

160, I think.

> you have already is still less than a cache line on
> some machines. Padding it to 256 bytes would make it nice and round,
> if you want to be really sure, make it 384 bytes.

Which we currently allocate on the kernel stack, plus up to a couple of kstat
structs if something like eCryptFS is used.  Admittedly, the base xstat struct
could be kmalloc()'d instead, but why use up all that space if you don't need


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