fscache: Implement cookie invalidation

Add a function to invalidate the cache behind a cookie:

	void fscache_invalidate(struct fscache_cookie *cookie,
				const void *aux_data,
				loff_t size,
				unsigned int flags)

This causes any cached data for the specified cookie to be discarded.  If
the cookie is marked as being in use, a new cache object will be created if
possible and future I/O will use that instead.  In-flight I/O should be
abandoned (writes) or reconsidered (reads).  Each time it is called
cookie->inval_counter is incremented and this can be used to detect
invalidation at the end of an I/O operation.

The coherency data attached to the cookie can be updated and the cookie
size should be reset.  One flag is available, FSCACHE_INVAL_DIO_WRITE,
which should be used to indicate invalidation due to a DIO write on a
file.  This will temporarily disable caching for this cookie.

Changes
=======
ver #2:
 - Should only change to inval state if can get access to cache.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819602231.215744.11206598147269491575.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906909707.143852.18056070560477964891.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967107447.1823006.5945029409592119962.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021512640.640689.11418616313147754172.stgit@warthog.procyon.org.uk/ # v4

1 files changed, 5 insertions, 0 deletions

diff --git a/fs/fscache/stats.c b/fs/fscache/stats.c
index 5aa4bd9fe207..cdbb672a274f 100644
--- a/fs/fscache/stats.c
+++ b/fs/fscache/stats.c
@@ -26,6 +26,8 @@ atomic_t fscache_n_acquires;
 atomic_t fscache_n_acquires_ok;
 atomic_t fscache_n_acquires_oom;
 
+atomic_t fscache_n_invalidates;
+
 atomic_t fscache_n_updates;
 EXPORT_SYMBOL(fscache_n_updates);
 
@@ -59,6 +61,9 @@ int fscache_stats_show(struct seq_file *m, void *v)
 		   timer_pending(&fscache_cookie_lru_timer) ?
 		   fscache_cookie_lru_timer.expires - jiffies : 0);
 
+	seq_printf(m, "Invals : n=%u\n",
+		   atomic_read(&fscache_n_invalidates));
+
 	seq_printf(m, "Updates: n=%u\n",
 		   atomic_read(&fscache_n_updates));