cachefiles: Implement backing file wrangling

Implement the wrangling of backing files, including the following pieces:

 (1) Lookup and creation of a file on disk, using a tmpfile if the file
     isn't yet present.  The file is then opened, sized for DIO and the
     file handle is attached to the cachefiles_object struct.  The inode is
     marked to indicate that it's in use by a kernel service.

 (2) Invalidation of an object, creating a tmpfile and switching the file
     pointer in the cachefiles object.

 (3) Committing a file to disk, including setting the coherency xattr on it
     and, if necessary, creating a hard link to it.

     Note that this would be a good place to use Omar Sandoval's vfs_link()
     with AT_LINK_REPLACE[1] as I may have to unlink an old file before I
     can link a tmpfile into place.

 (4) Withdrawal of open objects when a cache is being withdrawn or a cookie
     is relinquished.  This involves committing or discarding the file.

Changes
=======
ver #2:
 - Fix logging of wrong error[1].

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/20211203094950.GA2480@kili/ [1]
Link: https://lore.kernel.org/r/163819644097.215744.4505389616742411239.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906949512.143852.14222856795032602080.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967158526.1823006.17482695321424642675.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021557060.640689.16373541458119269871.stgit@warthog.procyon.org.uk/ # v4

1 files changed, 31 insertions, 1 deletions

diff --git a/fs/cachefiles/cache.c b/fs/cachefiles/cache.c
index c4b9280ca0cd..e2cbbc08bad9 100644
--- a/fs/cachefiles/cache.c
+++ b/fs/cachefiles/cache.c
@@ -263,6 +263,36 @@ begin_cull:
 }
 
 /*
+ * Mark all the objects as being out of service and queue them all for cleanup.
+ */
+static void cachefiles_withdraw_objects(struct cachefiles_cache *cache)
+{
+	struct cachefiles_object *object;
+	unsigned int count = 0;
+
+	_enter("");
+
+	spin_lock(&cache->object_list_lock);
+
+	while (!list_empty(&cache->object_list)) {
+		object = list_first_entry(&cache->object_list,
+					  struct cachefiles_object, cache_link);
+		cachefiles_see_object(object, cachefiles_obj_see_withdrawal);
+		list_del_init(&object->cache_link);
+		fscache_withdraw_cookie(object->cookie);
+		count++;
+		if ((count & 63) == 0) {
+			spin_unlock(&cache->object_list_lock);
+			cond_resched();
+			spin_lock(&cache->object_list_lock);
+		}
+	}
+
+	spin_unlock(&cache->object_list_lock);
+	_leave(" [%u objs]", count);
+}
+
+/*
  * Withdraw volumes.
  */
 static void cachefiles_withdraw_volumes(struct cachefiles_cache *cache)
@@ -326,7 +356,7 @@ void cachefiles_withdraw_cache(struct cachefiles_cache *cache)
 	/* we now have to destroy all the active objects pertaining to this
 	 * cache - which we do by passing them off to thread pool to be
 	 * disposed of */
-	// PLACEHOLDER: Withdraw objects
+	cachefiles_withdraw_objects(cache);
 	fscache_wait_for_objects(fscache);
 
 	cachefiles_withdraw_volumes(cache);