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

2 files changed, 43 insertions, 0 deletions

diff --git a/include/linux/fscache-cache.h b/include/linux/fscache-cache.h
index 3f0b19dcfae7..3235ddbdcc09 100644
--- a/include/linux/fscache-cache.h
+++ b/include/linux/fscache-cache.h
@@ -304,6 +304,10 @@ struct fscache_cache_ops {
 
 	/* dissociate a cache from all the pages it was backing */
 	void (*dissociate_pages)(struct fscache_cache *cache);
+
+	/* Begin a read operation for the netfs lib */
+	int (*begin_read_operation)(struct netfs_read_request *rreq,
+				    struct fscache_retrieval *op);
 };
 
 extern struct fscache_cookie fscache_fsdef_index;
diff --git a/include/linux/fscache.h b/include/linux/fscache.h
index 1f8dc72369ee..abc1c4737fb8 100644
--- a/include/linux/fscache.h
+++ b/include/linux/fscache.h
@@ -37,6 +37,7 @@ struct pagevec;
 struct fscache_cache_tag;
 struct fscache_cookie;
 struct fscache_netfs;
+struct netfs_read_request;
 
 typedef void (*fscache_rw_complete_t)(struct page *page,
 				      void *context,
@@ -191,6 +192,10 @@ extern void __fscache_update_cookie(struct fscache_cookie *, const void *);
 extern int __fscache_attr_changed(struct fscache_cookie *);
 extern void __fscache_invalidate(struct fscache_cookie *);
 extern void __fscache_wait_on_invalidate(struct fscache_cookie *);
+
+#ifdef FSCACHE_USE_NEW_IO_API
+extern int __fscache_begin_read_operation(struct netfs_read_request *, struct fscache_cookie *);
+#else
 extern int __fscache_read_or_alloc_page(struct fscache_cookie *,
 					struct page *,
 					fscache_rw_complete_t,
@@ -214,6 +219,8 @@ extern void __fscache_uncache_all_inode_pages(struct fscache_cookie *,
 					      struct inode *);
 extern void __fscache_readpages_cancel(struct fscache_cookie *cookie,
 				       struct list_head *pages);
+#endif /* FSCACHE_USE_NEW_IO_API */
+
 extern void __fscache_disable_cookie(struct fscache_cookie *, const void *, bool);
 extern void __fscache_enable_cookie(struct fscache_cookie *, const void *, loff_t,
 				    bool (*)(void *), void *);
@@ -498,6 +505,36 @@ int fscache_reserve_space(struct fscache_cookie *cookie, loff_t size)
 	return -ENOBUFS;
 }
 
+#ifdef FSCACHE_USE_NEW_IO_API
+
+/**
+ * fscache_begin_read_operation - Begin a read operation for the netfs lib
+ * @rreq: The read request being undertaken
+ * @cookie: The cookie representing the cache object
+ *
+ * Begin a read operation on behalf of the netfs helper library.  @rreq
+ * indicates the read request to which the operation state should be attached;
+ * @cookie indicates the cache object that will be accessed.
+ *
+ * This is intended to be called from the ->begin_cache_operation() netfs lib
+ * operation as implemented by the network filesystem.
+ *
+ * Returns:
+ * * 0		- Success
+ * * -ENOBUFS	- No caching available
+ * * Other error code from the cache, such as -ENOMEM.
+ */
+static inline
+int fscache_begin_read_operation(struct netfs_read_request *rreq,
+				 struct fscache_cookie *cookie)
+{
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
+		return __fscache_begin_read_operation(rreq, cookie);
+	return -ENOBUFS;
+}
+
+#else /* FSCACHE_USE_NEW_IO_API */
+
 /**
  * fscache_read_or_alloc_page - Read a page from the cache or allocate a block
  * in which to store it
@@ -777,6 +814,8 @@ void fscache_uncache_all_inode_pages(struct fscache_cookie *cookie,
 		__fscache_uncache_all_inode_pages(cookie, inode);
 }
 
+#endif /* FSCACHE_USE_NEW_IO_API */
+
 /**
  * fscache_disable_cookie - Disable a cookie
  * @cookie: The cookie representing the cache object