[patch] loop.c to use write ops for fs requiring special locking

[Robert S Peterson <rpeterso@redhat.com>]

Hi All,

Below is a small patch to loop.c I'd like to see in the kernel.

This is an extension of Anton Altaparmakov's previous fix which allows
loop.c to use the aop->write rather than prepare_write/commit_write if
prepare_write/commit_write aren't available.

Right now, the current loop.c uses aop->prepare_write/commit_write
unless there is no other option.  However, due to special locking
requirements, some backing filesystems may prefer the use of aop->write
rather than prepare_write/commit_write.  Since loop.c does not have
advisory locking, the backing fs should have a choice of which to use.

In the case of GFS, for example, loop.c's use of aop->prepare_write
circumvents proper cluster locking and transaction building, so using
aop->write is the right thing for loop.c to do.

How the patch works:
If the backing filesystem has special locking requirements (new flag in
fs_flags) loop.c uses aop->write rather than prepare_write/commit_write.

Feedback?

Regards,

Bob Peterson
rpeterso@redhat.com

--- linux-2.6.15.4.orig/drivers/block/loop.c    2006-02-10
01:22:48.000000000 -0600
+++ linux-2.6.15.4.patched/drivers/block/loop.c 2006-03-01
09:38:48.000000000 -0600
@@ -44,6 +44,11 @@
  * backing filesystem.
  * Anton Altaparmakov, 16 Feb 2005
  *
+ * Extension of Anton's idea: Use normal write file operations rather
+ * than prepare_write and commit_write when the backing filesystem
+ * requires special locking.
+ * Robert Peterson <rpeterso@redhat.com>, 01 Mar 2006
+ *
  * Still To Fix:
  * - Advisory locking is ignored here.
  * - Should use an own CAP_* category instead of CAP_SYS_ADMIN
@@ -74,6 +79,7 @@
 #include <linux/completion.h>
 #include <linux/highmem.h>
 #include <linux/gfp.h>
+#include <linux/mount.h>

 #include <asm/uaccess.h>

@@ -781,7 +787,8 @@ static int loop_set_fd(struct loop_devic
                 */
                if (!file->f_op->sendfile)
                        goto out_putf;
-               if (aops->prepare_write && aops->commit_write)
+               if (!(file->f_vfsmnt->mnt_sb->s_type->fs_flags &
FS_REQUIRES_LOCKING) &&
+                       aops->prepare_write && aops->commit_write)
                        lo_flags |= LO_FLAGS_USE_AOPS;
                if (!(lo_flags & LO_FLAGS_USE_AOPS) && !
file->f_op->write)
                        lo_flags |= LO_FLAGS_READ_ONLY;
diff -pur linux-2.6.15.4.orig/include/linux/fs.h
linux-2.6.15.4.patched/include/linux/fs.h
--- linux-2.6.15.4.orig/include/linux/fs.h      2006-02-10
01:22:48.000000000 -0600
+++ linux-2.6.15.4.patched/include/linux/fs.h   2006-02-28
17:18:48.000000000 -0600
@@ -83,6 +83,7 @@ extern int dir_notify_enable;
 /* public flags for file_system_type */
 #define FS_REQUIRES_DEV 1
 #define FS_BINARY_MOUNTDATA 2
+#define FS_REQUIRES_LOCKING 4   /* Filesystem requires locking */
 #define FS_REVAL_DOT   16384   /* Check the paths ".", ".." for
staleness */
 #define FS_ODD_RENAME  32768   /* Temporary stuff; will go away as soon
                                  * as nfs_rename() will be cleaned up