fix extent corruption in xfs_iext_irec_compact_full()

This function is used to compact the indirect extent list by moving extents from one page to the previous to fill them up. After we move some extents to an earlier page we need to shuffle the remaining extents to the start of the page. The actual bug here is the second argument to memmove() needs to index past the extents, that were copied to the previous page, and move the remaining extents. For pages that are already full (ie ext_avail == 0) the compaction code has no net effect so don't do it.

Thanks to Dave Chinner for pointing out the bug.

Quoted 9 lines · Lachlan McIlroy

This function is used to compact the indirect extent list by moving extents from one page to the previous to fill them up. After we move some extents to an earlier page we need to shuffle the remaining extents to the start of the page. The actual bug here is the second argument to memmove() needs to index past the extents, that were copied to the previous page, and move the remaining extents. For pages that are already full (ie ext_avail == 0) the compaction code has no net effect so don't do it.

Quoted 1 line

Thanks to Dave Chinner for pointing out the bug.

Looks good but this function needs a lot more comments. Below is a version of the patch with some more comments describing what's going on that I came up with when trying to understand what this patch does in detail:

Index: linux-2.6-xfs/fs/xfs/xfs_inode.c
===================================================================

fs/xfs/xfs_inode.c +47 -23

@@ -4532,39 +4532,63 @@ xfs_iext_irec_compact_full(
 	int		nlists;			/* number of irec's (ex lists) */
 
 	ASSERT(ifp->if_flags & XFS_IFEXTIREC);
+
 	nlists = ifp->if_real_bytes / XFS_IEXT_BUFSZ;
 	erp = ifp->if_u1.if_ext_irec;
 	ep = &erp->er_extbuf[erp->er_extcount];
 	erp_next = erp + 1;
 	ep_next = erp_next->er_extbuf;
+
 	while (erp_idx < nlists - 1) {
+		/*
+		 * Check how many extent records are available in this irec.
+		 * If there is none skip the whole exercise.
+		 */
 		ext_avail = XFS_LINEAR_EXTS - erp->er_extcount;
-		ext_diff = MIN(ext_avail, erp_next->er_extcount);
-		memcpy(ep, ep_next, ext_diff * sizeof(xfs_bmbt_rec_t));
-		erp->er_extcount += ext_diff;
-		erp_next->er_extcount -= ext_diff;
-		/* Remove next page */
-		if (erp_next->er_extcount == 0) {
+		if (ext_avail) {
+
+			/*
+			 * Copy over as many as possible extent records into
+			 * the previous page.
+			 */
+			ext_diff = MIN(ext_avail, erp_next->er_extcount);
+			memcpy(ep, ep_next, ext_diff * sizeof(xfs_bmbt_rec_t));
+			erp->er_extcount += ext_diff;
+			erp_next->er_extcount -= ext_diff;
+
 			/*
-			 * Free page before removing extent record
-			 * so er_extoffs don't get modified in
-			 * xfs_iext_irec_remove.
+			 * If the next irec is empty now we can simply
+			 * remove it.
 			 */
-			kmem_free(erp_next->er_extbuf);
-			erp_next->er_extbuf = NULL;
-			xfs_iext_irec_remove(ifp, erp_idx + 1);
-			erp = &ifp->if_u1.if_ext_irec[erp_idx];
-			nlists = ifp->if_real_bytes / XFS_IEXT_BUFSZ;
-		/* Update next page */
-		} else {
-			/* Move rest of page up to become next new page */
-			memmove(erp_next->er_extbuf, ep_next,
-				erp_next->er_extcount * sizeof(xfs_bmbt_rec_t));
-			ep_next = erp_next->er_extbuf;
-			memset(&ep_next[erp_next->er_extcount], 0,
-				(XFS_LINEAR_EXTS - erp_next->er_extcount) *
-				sizeof(xfs_bmbt_rec_t));
+			if (erp_next->er_extcount == 0) {
+				/*
+				 * Free page before removing extent record
+				 * so er_extoffs don't get modified in
+				 * xfs_iext_irec_remove.
+				 */
+				kmem_free(erp_next->er_extbuf);
+				erp_next->er_extbuf = NULL;
+				xfs_iext_irec_remove(ifp, erp_idx + 1);
+				erp = &ifp->if_u1.if_ext_irec[erp_idx];
+				nlists = ifp->if_real_bytes / XFS_IEXT_BUFSZ;
+
+			/*
+			 * If the next irec is not empty move up the content
+			 * that has not been copied to the previous page to
+			 * the beggining of this one.
+			 */
+			} else {
+				memmove(erp_next->er_extbuf, &ep_next[ext_diff],
+					erp_next->er_extcount *
+					sizeof(xfs_bmbt_rec_t));
+				ep_next = erp_next->er_extbuf;
+				memset(&ep_next[erp_next->er_extcount], 0,
+					(XFS_LINEAR_EXTS -
+						erp_next->er_extcount) *
+					sizeof(xfs_bmbt_rec_t));
+			}
 		}
+
 		if (erp->er_extcount == XFS_LINEAR_EXTS) {
 			erp_idx++;
 			if (erp_idx < nlists)