xfs: add buffer pre-write callback

Add a callback to the buffer write path to enable verification of
the buffer and CRC calculation prior to issuing the write to the
underlying storage.

If the callback function detects some kind of failure or error
condition, it must mark the buffer with an error so that the caller
can take appropriate action. In the case of xfs_buf_ioapply(), a
corrupt metadta buffer willt rigger a shutdown of the filesystem,
because something is clearly wrong and we can't allow corrupt
metadata to be written to disk.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Phil White <pwhite@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

2 files changed, 19 insertions, 0 deletions

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index fbc965fc075a..bd1a948ee39c 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -569,7 +569,9 @@ found:
          */
         if (bp->b_flags & XBF_STALE) {
                 ASSERT((bp->b_flags & _XBF_DELWRI_Q) == 0);
+                ASSERT(bp->b_iodone == NULL);
                 bp->b_flags &= _XBF_KMEM | _XBF_PAGES;
+                bp->b_pre_io = NULL;
         }
 
         trace_xfs_buf_find(bp, flags, _RET_IP_);
@@ -1324,6 +1326,20 @@ _xfs_buf_ioapply(
         rw |= REQ_META;
 
         /*
+         * run the pre-io callback function if it exists. If this function
+         * fails it will mark the buffer with an error and the IO should
+         * not be dispatched.
+         */
+        if (bp->b_pre_io) {
+                bp->b_pre_io(bp);
+                if (bp->b_error) {
+                        xfs_force_shutdown(bp->b_target->bt_mount,
+                                           SHUTDOWN_CORRUPT_INCORE);
+                        return;
+                }
+        }
+
+        /*
          * Walk all the vectors issuing IO on them. Set up the initial offset
          * into the buffer and the desired IO size before we start -
          * _xfs_buf_ioapply_vec() will modify them appropriately for each
diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
index 677b1dc822f4..51bc16a1cd9c 100644
--- a/fs/xfs/xfs_buf.h
+++ b/fs/xfs/xfs_buf.h
@@ -155,6 +155,9 @@ typedef struct xfs_buf {
         unsigned int            b_offset;       /* page offset in first page */
         unsigned short          b_error;        /* error code on I/O */
 
+        void                    (*b_pre_io)(struct xfs_buf *);
+                                                /* pre-io callback function */
+
 #ifdef XFS_BUF_LOCK_TRACKING
         int                     b_last_holder;
 #endif