fscache, cachefiles: Add alternate API to use kiocb for read/write to cache

Add an alternate API by which the cache can be accessed through a kiocb,
doing async DIO, rather than using the current API that tells the cache
where all the pages are.

The new API is intended to be used in conjunction with the netfs helper
library.  A filesystem must pick one or the other and not mix them.

Filesystems wanting to use the new API must #define FSCACHE_USE_NEW_IO_API
before #including the header.  This prevents them from continuing to use
the old API at the same time as there are incompatibilities in how the
PG_fscache page bit is used.

Changes:
v6:
 - Provide a routine to shape a write so that the start and length can be
   aligned for DIO[3].

v4:
 - Use the vfs_iocb_iter_read/write() helpers[1]
 - Move initial definition of fscache_begin_read_operation() here.
 - Remove a commented-out line[2]
 - Combine ki->term_func calls in cachefiles_read_complete()[2].
 - Remove explicit NULL initialiser[2].
 - Remove extern on func decl[2].
 - Put in param names on func decl[2].
 - Remove redundant else[2].
 - Fill out the kdoc comment for fscache_begin_read_operation().
 - Rename fs/fscache/page2.c to io.c to match later patches.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-and-tested-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Dave Wysochanski <dwysocha@redhat.com>
Tested-By: Marc Dionne <marc.dionne@auristor.com>
cc: Christoph Hellwig <hch@lst.de>
cc: linux-cachefs@redhat.com
cc: linux-afs@lists.infradead.org
cc: linux-nfs@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: ceph-devel@vger.kernel.org
cc: v9fs-developer@lists.sourceforge.net
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20210216102614.GA27555@lst.de/ [1]
Link: https://lore.kernel.org/r/20210216084230.GA23669@lst.de/ [2]
Link: https://lore.kernel.org/r/161781047695.463527.7463536103593997492.stgit@warthog.procyon.org.uk/ [3]
Link: https://lore.kernel.org/r/161118142558.1232039.17993829899588971439.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/161161037850.2537118.8819808229350326503.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/161340402057.1303470.8038373593844486698.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/161539545919.286939.14573472672781434757.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/161653801477.2770958.10543270629064934227.stgit@warthog.procyon.org.uk/ # v5
Link: https://lore.kernel.org/r/161789084517.6155.12799689829859169640.stgit@warthog.procyon.org.uk/ # v6

1 files changed, 9 insertions, 0 deletions

diff --git a/fs/cachefiles/internal.h b/fs/cachefiles/internal.h
index cf9bd6401c2d..4ed83aa5253b 100644
--- a/fs/cachefiles/internal.h
+++ b/fs/cachefiles/internal.h
@@ -150,6 +150,9 @@ extern int cachefiles_has_space(struct cachefiles_cache *cache,
  */
 extern const struct fscache_cache_ops cachefiles_cache_ops;
 
+void cachefiles_put_object(struct fscache_object *_object,
+			   enum fscache_obj_ref_trace why);
+
 /*
  * key.c
  */
@@ -218,6 +221,12 @@ extern int cachefiles_write_page(struct fscache_storage *, struct page *);
 extern void cachefiles_uncache_page(struct fscache_object *, struct page *);
 
 /*
+ * rdwr2.c
+ */
+extern int cachefiles_begin_read_operation(struct netfs_read_request *,
+					   struct fscache_retrieval *);
+
+/*
  * security.c
  */
 extern int cachefiles_get_security_ID(struct cachefiles_cache *cache);