[spruce-project] [PATCH] ext4: implement error handling of ext4_mb_new_preallocation()

[Alexey Khoroshilov]

If memory allocation in ext4_mb_new_group_pa() is failed,
it returns error code, ext4_mb_new_preallocation() propages it,
but ext4_mb_new_blocks() ignores it.

An observed result was:
- allocation fail means ext4_mb_new_group_pa() does not update ext4_allocation_context;
- ext4_mb_new_blocks() sets ext4_allocation_request->len (ar->len = ac->ac_b_ex.fe_len;)
  to number of blocks preallocated (512) instead of number of blocks requested (1);
- that activates update cycle in ext4_splice_branch():
    for (i = 1; i < blks; i++) <-- blks is 512 instead of 1 here
      *(where->p + i) = cpu_to_le32(current_block++);
- it iterates 511 times and corrupts a chunk of memory including inode structure;
- page fault happens at EXT4_SB(inode->i_sb) in ext4_mark_inode_dirty();
- system hangs with 'scheduling while atomic' BUG.

The patch implements a check for ext4_mb_new_preallocation() error code
and handles its failure as if ext4_mb_regular_allocator() fails.

Found by Linux File System Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov at ispras.ru>
---
 fs/ext4/mballoc.c |   15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index b1ed9e0..95c6b0d 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4401,17 +4401,18 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t *handle,
 repeat:
 		/* allocate space in core */
 		*errp = ext4_mb_regular_allocator(ac);
+		if (!*errp) {
+			/* as we've just preallocated more space than
+			 * user requested orinally, we store allocated
+			 * space in a special descriptor */
+			if (ac->ac_status == AC_STATUS_FOUND &&
+					ac->ac_o_ex.fe_len < ac->ac_b_ex.fe_len)
+				*errp = ext4_mb_new_preallocation(ac);
+		}
 		if (*errp) {
 			ext4_discard_allocated_blocks(ac);
 			goto errout;
 		}
-
-		/* as we've just preallocated more space than
-		 * user requested orinally, we store allocated
-		 * space in a special descriptor */
-		if (ac->ac_status == AC_STATUS_FOUND &&
-				ac->ac_o_ex.fe_len < ac->ac_b_ex.fe_len)
-			ext4_mb_new_preallocation(ac);
 	}
 	if (likely(ac->ac_status == AC_STATUS_FOUND)) {
 		*errp = ext4_mb_mark_diskspace_used(ac, handle, reserv_clstrs);
-- 
1.7.9.5