[Linux-cachefs] Finding fscache contents for a file

[John Groves]

I have modified the lustre client filesystem to use fscache, and it is in a
rudimentary working state.  Among my most pressing requirements is to purge
the fscache for any extent for which a DLM lock is revoked.

To the end of proving that functionality, I would like to give myself a file
ioctl that would determine what is in the fscache for a given file.  Since
this is for testing, performance isn't a major concern.  I'm already doing
this with the page cache, and I hope something similar would be possible
with the fscache.

Is there a supported way to query whether a given page_index is in the
fscache?  If not, I'd appreciate suggestions as to how to go about this (or
insight into how other implementers have proven functionality without this
feature).  I'm fairly ignorant as to the internals of fscache...

Thanks,
John Groves