ext4: refresh the ext4_ext_path struct after dropping i_data_sem.

After we drop i_data sem, we need to reload the ext4_ext_path
structure since the extent tree can change once i_data_sem is
released.

This addresses the BUG:

[52117.465187] ------------[ cut here ]------------
[52117.465686] kernel BUG at fs/ext4/extents.c:1756!
...
[52117.478306] Call Trace:
[52117.478565]  ext4_ext_shift_extents+0x3ee/0x710
[52117.479020]  ext4_fallocate+0x139c/0x1b40
[52117.479405]  ? __do_sys_newfstat+0x6b/0x80
[52117.479805]  vfs_fallocate+0x151/0x4b0
[52117.480177]  ksys_fallocate+0x4a/0xa0
[52117.480533]  __x64_sys_fallocate+0x22/0x30
[52117.480930]  do_syscall_64+0x35/0x80
[52117.481277]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[52117.481769] RIP: 0033:0x7fa062f855ca

Cc: stable@kernel.org
Link: https://lore.kernel.org/r/20210903062748.4118886-4-yangerkun@huawei.comSigned-off-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/extents.c

@@ -5014,8 +5014,11 @@ ext4_ext_shift_path_extents(struct ext4_ext_path *path, ext4_lblk_t shift,
 			restart_credits = ext4_writepage_trans_blocks(inode);
 			err = ext4_datasem_ensure_credits(handle, inode, credits,
 					restart_credits, 0);
-			if (err)
+			if (err) {
+				if (err > 0)
+					err = -EAGAIN;
 				goto out;
+			}
 
 			err = ext4_ext_get_access(handle, inode, path + depth);
 			if (err)
@@ -5089,6 +5092,7 @@ ext4_ext_shift_extents(struct inode *inode, handle_t *handle,
 	int ret = 0, depth;
 	struct ext4_extent *extent;
 	ext4_lblk_t stop, *iterator, ex_start, ex_end;
+	ext4_lblk_t tmp = EXT_MAX_BLOCKS;
 
 	/* Let path point to the last extent */
 	path = ext4_find_extent(inode, EXT_MAX_BLOCKS - 1, NULL,
@@ -5142,11 +5146,15 @@ ext4_ext_shift_extents(struct inode *inode, handle_t *handle,
 	 * till we reach stop. In case of right shift, iterator points to stop
 	 * and it is decreased till we reach start.
 	 */
+again:
 	if (SHIFT == SHIFT_LEFT)
 		iterator = &start;
 	else
 		iterator = &stop;
 
+	if (tmp != EXT_MAX_BLOCKS)
+		*iterator = tmp;
+
 	/*
 	 * Its safe to start updating extents.  Start and stop are unsigned, so
 	 * in case of right shift if extent with 0 block is reached, iterator
@@ -5175,6 +5183,7 @@ ext4_ext_shift_extents(struct inode *inode, handle_t *handle,
 			}
 		}
 
+		tmp = *iterator;
 		if (SHIFT == SHIFT_LEFT) {
 			extent = EXT_LAST_EXTENT(path[depth].p_hdr);
 			*iterator = le32_to_cpu(extent->ee_block) +
@@ -5193,6 +5202,9 @@ ext4_ext_shift_extents(struct inode *inode, handle_t *handle,
 		}
 		ret = ext4_ext_shift_path_extents(path, shift, inode,
 				handle, SHIFT);
+		/* iterator can be NULL which means we should break */
+		if (ret == -EAGAIN)
+			goto again;
 		if (ret)
 			break;
 	}