fscache, cachefiles: Display stat of culling events

Add a stat counter of culling events whereby the cache backend culls a file
to make space (when asked by cachefilesd in this case) and display in
/proc/fs/fscache/stats.

Signed-off-by: David Howells <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>
cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]/ # v1
Link: https://lore.kernel.org/r/[e-mail]/ # v2
Link: https://lore.kernel.org/r/[e-mail]/ # v3
Link: https://lore.kernel.org/r/[e-mail]/ # v4

fs/cachefiles/namei.c

···
 	if (ret < 0)
 		goto error;
 
+	fscache_count_culled();
 	dput(victim);
 	_leave(" = 0");
 	return 0;

fs/fscache/stats.c

···
 EXPORT_SYMBOL(fscache_n_no_write_space);
 atomic_t fscache_n_no_create_space;
 EXPORT_SYMBOL(fscache_n_no_create_space);
+atomic_t fscache_n_culled;
+EXPORT_SYMBOL(fscache_n_culled);
 
 /*
  * display the general statistics
···
 		   atomic_read(&fscache_n_relinquishes_retire),
 		   atomic_read(&fscache_n_relinquishes_dropped));
 
-	seq_printf(m, "NoSpace: nwr=%u ncr=%u\n",
+	seq_printf(m, "NoSpace: nwr=%u ncr=%u cull=%u\n",
 		   atomic_read(&fscache_n_no_write_space),
-		   atomic_read(&fscache_n_no_create_space));
+		   atomic_read(&fscache_n_no_create_space),
+		   atomic_read(&fscache_n_culled));
 
 	seq_printf(m, "IO     : rd=%u wr=%u\n",
 		   atomic_read(&fscache_n_read),

include/linux/fscache-cache.h

···
 extern atomic_t fscache_n_write;
 extern atomic_t fscache_n_no_write_space;
 extern atomic_t fscache_n_no_create_space;
+extern atomic_t fscache_n_culled;
 #define fscache_count_read() atomic_inc(&fscache_n_read)
 #define fscache_count_write() atomic_inc(&fscache_n_write)
 #define fscache_count_no_write_space() atomic_inc(&fscache_n_no_write_space)
 #define fscache_count_no_create_space() atomic_inc(&fscache_n_no_create_space)
+#define fscache_count_culled() atomic_inc(&fscache_n_culled)
 #else
 #define fscache_count_read() do {} while(0)
 #define fscache_count_write() do {} while(0)
 #define fscache_count_no_write_space() do {} while(0)
 #define fscache_count_no_create_space() do {} while(0)
+#define fscache_count_culled() do {} while(0)
 #endif
 
 #endif /* _LINUX_FSCACHE_CACHE_H */