Re: [PATCH] ext4: implement error handling ofext4_mb_new_preallocation()

From: Theodore Ts'o
Date: Mon Jun 17 2013 - 09:17:55 EST


On Mon, May 06, 2013 at 12:51:15AM +0400, Alexey Khoroshilov wrote:
> 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@xxxxxxxxx>

Hi Alexey,

Thanks for reporting this bug, and proposing a fix. I've restructured
the patch slightly to make the flow of control slightly easier to
follow, and more consistent with the coding style in ext4.

Thanks again!

- Ted