target/file: Re-enable optional fd_buffered_io=1 operation

This patch re-adds the ability to optionally run in buffered FILEIO mode
(eg: w/o O_DSYNC) for device backends in order to once again use the
Linux buffered cache as a write-back storage mechanism.

This logic was originally dropped with mainline v3.5-rc commit:

commit a4dff3043c231d57f982af635c9d2192ee40e5ae
Author: Nicholas Bellinger <nab@linux-iscsi.org>
Date:   Wed May 30 16:25:41 2012 -0700

    target/file: Use O_DSYNC by default for FILEIO backends

This difference with this patch is that fd_create_virtdevice() now
forces the explicit setting of emulate_write_cache=1 when buffered FILEIO
operation has been enabled.

(v2: Switch to FDBD_HAS_BUFFERED_IO_WCE + add more detailed
     comment as requested by hch)

Reported-by: Ferry <iscsitmp@bananateam.nl>
Cc: Christoph Hellwig <hch@lst.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

2 files changed, 39 insertions, 3 deletions

diff --git a/drivers/target/target_core_file.c b/drivers/target/target_core_file.c
index 11119257e514..0360383dfb94 100644
--- a/drivers/target/target_core_file.c
+++ b/drivers/target/target_core_file.c
@@ -125,6 +125,19 @@ static struct se_device *fd_create_virtdevice(
          * of pure timestamp updates.
          */
         flags = O_RDWR | O_CREAT | O_LARGEFILE | O_DSYNC;
+        /*
+         * Optionally allow fd_buffered_io=1 to be enabled for people
+         * who want use the fs buffer cache as an WriteCache mechanism.
+         *
+         * This means that in event of a hard failure, there is a risk
+         * of silent data-loss if the SCSI client has *not* performed a
+         * forced unit access (FUA) write, or issued SYNCHRONIZE_CACHE
+         * to write-out the entire device cache.
+         */
+        if (fd_dev->fbd_flags & FDBD_HAS_BUFFERED_IO_WCE) {
+                pr_debug("FILEIO: Disabling O_DSYNC, using buffered FILEIO\n");
+                flags &= ~O_DSYNC;
+        }
 
         file = filp_open(fd_dev->fd_dev_name, flags, 0600);
         if (IS_ERR(file)) {
@@ -188,6 +201,12 @@ static struct se_device *fd_create_virtdevice(
         if (!dev)
                 goto fail;
 
+        if (fd_dev->fbd_flags & FDBD_HAS_BUFFERED_IO_WCE) {
+                pr_debug("FILEIO: Forcing setting of emulate_write_cache=1"
+                        " with FDBD_HAS_BUFFERED_IO_WCE\n");
+                dev->se_sub_dev->se_dev_attrib.emulate_write_cache = 1;
+        }
+
         fd_dev->fd_dev_id = fd_host->fd_host_dev_id_count++;
         fd_dev->fd_queue_depth = dev->queue_depth;
 
@@ -407,6 +426,7 @@ enum {
 static match_table_t tokens = {
         {Opt_fd_dev_name, "fd_dev_name=%s"},
         {Opt_fd_dev_size, "fd_dev_size=%s"},
+        {Opt_fd_buffered_io, "fd_buffered_io=%d"},
         {Opt_err, NULL}
 };
 
@@ -418,7 +438,7 @@ static ssize_t fd_set_configfs_dev_params(
         struct fd_dev *fd_dev = se_dev->se_dev_su_ptr;
         char *orig, *ptr, *arg_p, *opts;
         substring_t args[MAX_OPT_ARGS];
-        int ret = 0, token;
+        int ret = 0, arg, token;
 
         opts = kstrdup(page, GFP_KERNEL);
         if (!opts)
@@ -459,6 +479,19 @@ static ssize_t fd_set_configfs_dev_params(
                                         " bytes\n", fd_dev->fd_dev_size);
                         fd_dev->fbd_flags |= FBDF_HAS_SIZE;
                         break;
+                case Opt_fd_buffered_io:
+                        match_int(args, &arg);
+                        if (arg != 1) {
+                                pr_err("bogus fd_buffered_io=%d value\n", arg);
+                                ret = -EINVAL;
+                                goto out;
+                        }
+
+                        pr_debug("FILEIO: Using buffered I/O"
+                                " operations for struct fd_dev\n");
+
+                        fd_dev->fbd_flags |= FDBD_HAS_BUFFERED_IO_WCE;
+                        break;
                 default:
                         break;
                 }
@@ -490,8 +523,10 @@ static ssize_t fd_show_configfs_dev_params(
         ssize_t bl = 0;
 
         bl = sprintf(b + bl, "TCM FILEIO ID: %u", fd_dev->fd_dev_id);
-        bl += sprintf(b + bl, "        File: %s  Size: %llu  Mode: O_DSYNC\n",
-                fd_dev->fd_dev_name, fd_dev->fd_dev_size);
+        bl += sprintf(b + bl, "        File: %s  Size: %llu  Mode: %s\n",
+                fd_dev->fd_dev_name, fd_dev->fd_dev_size,
+                (fd_dev->fbd_flags & FDBD_HAS_BUFFERED_IO_WCE) ?
+                "Buffered-WCE" : "O_DSYNC");
         return bl;
 }
 
diff --git a/drivers/target/target_core_file.h b/drivers/target/target_core_file.h
index 70ce7fd7111d..876ae53ef5b8 100644
--- a/drivers/target/target_core_file.h
+++ b/drivers/target/target_core_file.h
@@ -14,6 +14,7 @@
 
 #define FBDF_HAS_PATH           0x01
 #define FBDF_HAS_SIZE           0x02
+#define FDBD_HAS_BUFFERED_IO_WCE 0x04
 
 struct fd_dev {
         u32             fbd_flags;