Re: [PATCH v2 08/20] ovl: lookup index entry for non-dir

On Fri, Jun 9, 2017 at 4:14 PM, Miklos Szeredi [off-list ref] wrote:

On Fri, Jun 9, 2017 at 1:49 PM, Miklos Szeredi [off-list ref] wrote:

quoted

On Fri, Jun 9, 2017 at 11:38 AM, Amir Goldstein [off-list ref] wrote:

quoted

The challenge is in accounting the number of link-ups atomically with
the link-up itself.  No ideas off-hand.

Following would work I think:

- copy up to indexdir (it now has st_nlink == 1)
- set overlay.nlinkup to "1-2"
  + the first number indicates the target number of linkups
  + the second indicates the target st_nlink on the file
- fsync the file (hopefully this writes the xattrs as well as the data)
- link the file from indexdir to upper
- set overlay.nlinkup to "1"
   + this indicates that the linkup was finished and number of linkups is 1.

There are 3 cases when we find a file with overlay.nlinkup:

- just one number: nothing to do
- two numbers and second number == st_nlink: replace with just the linkup value
- two numbers and second number != st_nlink: replace with decremented
linkup value

So the atomicity is guaranteed by st_nlink becoming in sync with the
target value.

Would need to ensure that ovl_link() is not performed in parallel with
the linkup procedure.

Yep, I started to write you the same thing :)
I though of using a separate xattr for the transnational values,
and delete it after link up, but I guess single xattr is cleaner ??

I guess the new ovl_inode mutex could be used for blocking ovl_link().

Just to be clear:
- we are going to allocate ovl_inode from our own slab cache
- move from using the i_private inode fields to ovl_inode fields:
  {realinode, originode, nlinkup, lock}
- use ovl_inode mutex instead of wait queue and for hardlink up

Correct?

Amir.