FS-Cache: Provide the ability to enable/disable cookies

Provide the ability to enable and disable fscache cookies.  A disabled cookie
will reject or ignore further requests to:

	Acquire a child cookie
	Invalidate and update backing objects
	Check the consistency of a backing object
	Allocate storage for backing page
	Read backing pages
	Write to backing pages

but still allows:

	Checks/waits on the completion of already in-progress objects
	Uncaching of pages
	Relinquishment of cookies

Two new operations are provided:

 (1) Disable a cookie:

	void fscache_disable_cookie(struct fscache_cookie *cookie,
				    bool invalidate);

     If the cookie is not already disabled, this locks the cookie against other
     dis/enablement ops, marks the cookie as being disabled, discards or
     invalidates any backing objects and waits for cessation of activity on any
     associated object.

     This is a wrapper around a chunk split out of fscache_relinquish_cookie(),
     but it reinitialises the cookie such that it can be reenabled.

     All possible failures are handled internally.  The caller should consider
     calling fscache_uncache_all_inode_pages() afterwards to make sure all page
     markings are cleared up.

 (2) Enable a cookie:

	void fscache_enable_cookie(struct fscache_cookie *cookie,
				   bool (*can_enable)(void *data),
				   void *data)

     If the cookie is not already enabled, this locks the cookie against other
     dis/enablement ops, invokes can_enable() and, if the cookie is not an
     index cookie, will begin the procedure of acquiring backing objects.

     The optional can_enable() function is passed the data argument and returns
     a ruling as to whether or not enablement should actually be permitted to
     begin.

     All possible failures are handled internally.  The cookie will only be
     marked as enabled if provisional backing objects are allocated.

A later patch will introduce these to NFS.  Cookie enablement during nfs_open()
is then contingent on i_writecount <= 0.  can_enable() checks for a race
between open(O_RDONLY) and open(O_WRONLY/O_RDWR).  This simplifies NFS's cookie
handling and allows us to get rid of open(O_RDONLY) accidentally introducing
caching to an inode that's open for writing already.

One operation has its API modified:

 (3) Acquire a cookie.

	struct fscache_cookie *fscache_acquire_cookie(
		struct fscache_cookie *parent,
		const struct fscache_cookie_def *def,
		void *netfs_data,
		bool enable);

     This now has an additional argument that indicates whether the requested
     cookie should be enabled by default.  It doesn't need the can_enable()
     function because the caller must prevent multiple calls for the same netfs
     object and it doesn't need to take the enablement lock because no one else
     can get at the cookie before this returns.

Signed-off-by: David Howells <dhowells@redhat.com

1 files changed, 60 insertions, 13 deletions

diff --git a/Documentation/filesystems/caching/netfs-api.txt b/Documentation/filesystems/caching/netfs-api.txt
index 11a0a40ce445..aed6b94160b1 100644
--- a/Documentation/filesystems/caching/netfs-api.txt
+++ b/Documentation/filesystems/caching/netfs-api.txt
@@ -29,15 +29,16 @@ This document contains the following sections:
          (6) Index registration
          (7) Data file registration
          (8) Miscellaneous object registration
-         (9) Setting the data file size
+         (9) Setting the data file size
         (10) Page alloc/read/write
         (11) Page uncaching
         (12) Index and data file consistency
-        (13) Miscellaneous cookie operations
-        (14) Cookie unregistration
-        (15) Index invalidation
-        (16) Data file invalidation
-        (17) FS-Cache specific page flags.
+        (13) Cookie enablement
+        (14) Miscellaneous cookie operations
+        (15) Cookie unregistration
+        (16) Index invalidation
+        (17) Data file invalidation
+        (18) FS-Cache specific page flags.
 
 
 =============================
@@ -334,7 +335,8 @@ the path to the file:
         struct fscache_cookie *
         fscache_acquire_cookie(struct fscache_cookie *parent,
                                const struct fscache_object_def *def,
-                               void *netfs_data);
+                               void *netfs_data,
+                               bool enable);
 
 This function creates an index entry in the index represented by parent,
 filling in the index entry by calling the operations pointed to by def.
@@ -350,6 +352,10 @@ object needs to be created somewhere down the hierarchy.  Furthermore, an index
 may be created in several different caches independently at different times.
 This is all handled transparently, and the netfs doesn't see any of it.
 
+A cookie will be created in the disabled state if enabled is false.  A cookie
+must be enabled to do anything with it.  A disabled cookie can be enabled by
+calling fscache_enable_cookie() (see below).
+
 For example, with AFS, a cell would be added to the primary index.  This index
 entry would have a dependent inode containing a volume location index for the
 volume mappings within this cell:
@@ -357,7 +363,7 @@ volume mappings within this cell:
         cell->cache =
                 fscache_acquire_cookie(afs_cache_netfs.primary_index,
                                        &afs_cell_cache_index_def,
-                                       cell);
+                                       cell, true);
 
 Then when a volume location was accessed, it would be entered into the cell's
 index and an inode would be allocated that acts as a volume type and hash chain
@@ -366,7 +372,7 @@ combination:
         vlocation->cache =
                 fscache_acquire_cookie(cell->cache,
                                        &afs_vlocation_cache_index_def,
-                                       vlocation);
+                                       vlocation, true);
 
 And then a particular flavour of volume (R/O for example) could be added to
 that index, creating another index for vnodes (AFS inode equivalents):
@@ -374,7 +380,7 @@ that index, creating another index for vnodes (AFS inode equivalents):
         volume->cache =
                 fscache_acquire_cookie(vlocation->cache,
                                        &afs_volume_cache_index_def,
-                                       volume);
+                                       volume, true);
 
 
 ======================
@@ -388,7 +394,7 @@ the object definition should be something other than index type.
         vnode->cache =
                 fscache_acquire_cookie(volume->cache,
                                        &afs_vnode_cache_object_def,
-                                       vnode);
+                                       vnode, true);
 
 
 =================================
@@ -404,7 +410,7 @@ it would be some other type of object such as a data file.
         xattr->cache =
                 fscache_acquire_cookie(vnode->cache,
                                        &afs_xattr_cache_object_def,
-                                       xattr);
+                                       xattr, true);
 
 Miscellaneous objects might be used to store extended attributes or directory
 entries for example.
@@ -733,6 +739,47 @@ Note that partial updates may happen automatically at other times, such as when
 data blocks are added to a data file object.
 
 
+=================
+COOKIE ENABLEMENT
+=================
+
+Cookies exist in one of two states: enabled and disabled.  If a cookie is
+disabled, it ignores all attempts to acquire child cookies; check, update or
+invalidate its state; allocate, read or write backing pages - though it is
+still possible to uncache pages and relinquish the cookie.
+
+The initial enablement state is set by fscache_acquire_cookie(), but the cookie
+can be enabled or disabled later.  To disable a cookie, call:
+    
+        void fscache_disable_cookie(struct fscache_cookie *cookie,
+                                    bool invalidate);
+    
+If the cookie is not already disabled, this locks the cookie against other
+enable and disable ops, marks the cookie as being disabled, discards or
+invalidates any backing objects and waits for cessation of activity on any
+associated object before unlocking the cookie.
+
+All possible failures are handled internally.  The caller should consider
+calling fscache_uncache_all_inode_pages() afterwards to make sure all page
+markings are cleared up.
+    
+Cookies can be enabled or reenabled with:
+    
+        void fscache_enable_cookie(struct fscache_cookie *cookie,
+                                   bool (*can_enable)(void *data),
+                                   void *data)
+    
+If the cookie is not already enabled, this locks the cookie against other
+enable and disable ops, invokes can_enable() and, if the cookie is not an index
+cookie, will begin the procedure of acquiring backing objects.
+
+The optional can_enable() function is passed the data argument and returns a
+ruling as to whether or not enablement should actually be permitted to begin.
+
+All possible failures are handled internally.  The cookie will only be marked
+as enabled if provisional backing objects are allocated.
+
+
 ===============================
 MISCELLANEOUS COOKIE OPERATIONS
 ===============================
@@ -778,7 +825,7 @@ COOKIE UNREGISTRATION
 To get rid of a cookie, this function should be called.
 
         void fscache_relinquish_cookie(struct fscache_cookie *cookie,
-                                       int retire);
+                                       bool retire);
 
 If retire is non-zero, then the object will be marked for recycling, and all
 copies of it will be removed from all active caches in which it is present.