[Linux-cachefs] [BUG] write but no read when using sync mount option

[Chris Chilvers]

While trying the new FS-Cache implementation using the 5.17-rc5 kernel on
Ubuntu 21.10 I ran into an issue where it appears that FS-Cache was not being
used when the sync mount option is enabled.

While monitoring /proc/fs/fscache/stats it was observed there were high writes
to the cache volume, but no reads:

    IO     : rd=0 wr=344713665

Further tracing of /sys/kernel/debug/tracing/events/fscache/fscache_access shows
a large number of io_write operations, but no io_read operations.

The volume did fill up and perform culling as expected.

When the mount option was changed from sync to async the cache performed as
expected and a mixture of io_write and io_read was observed.

-- Chris