Re: [RFC][PATCH 0/5] vfs, overlayfs, cachefiles: Combine I_OVL_INUSE and S_KERNEL_FILE and split out no-remove

[Amir Goldstein <amir73il@gmail.com>]

On Mon, Jan 31, 2022 at 5:12 PM David Howells <dhowells@redhat.com> wrote:
>
>
> Hi Amir,
>
> How about this as a set of patches to do what you suggest[1] and hoist the
> handler functions for I_OVL_INUSE into common code and rename the flag to
> I_EXCL_INUSE.  This can then be shared with cachefiles - allowing me to get
> rid of S_KERNEL_FILE.
>

They look like what I had in mind.
Unfortunately, I had forgotten about another use that ovl makes of the flag
(see comment on patch 1/5). I'd made a suggestion on how to get rid of that use
case, but I hope this won't complicate things too much for you.

> I did split out the functionality for preventing file/dir removal to a
> separate flag, I_NO_REMOVE, so that it's not tied to I_EXCL_INUSE in case
> overlayfs doesn't want to use it.  The downside to that, though is that it
> requires a separate locking of i_lock to set/clear it.
>
> I also added four general tracepoints to log successful lock/unlock,
> failure to lock and a bad unlock.  The lock tracepoints log which driver
> asked for the lock and all tracepoints allow the driver to log an arbitrary
> reference number (in cachefiles's case this is the object debug ID).
>
> Questions:
>
>  (1) Should it be using a flag in i_state or a flag in i_flags?  I'm not
>      sure what the difference is really.

Me neither.

>
>  (2) Do we really need to take i_lock when testing I_EXCL_INUSE?  Would
>      READ_ONCE() suffice?
>

For ovl_is_inuse() I think READ_ONCE() should suffice.

Thanks,
Amir.