Re: [PATCH] ext4: prevent the normalized size from exceeding EXT_MAX_BLOCKS

[Jan Kara]

On Mon 27-11-23 14:33:13, Baokun Li wrote:
> For files with logical blocks close to EXT_MAX_BLOCKS, the file size
> predicted in ext4_mb_normalize_request() may exceed EXT_MAX_BLOCKS.
> This can cause some blocks to be preallocated that will not be used.
> And after [Fixes], the following issue may be triggered:
> 
> =========================================================
>  kernel BUG at fs/ext4/mballoc.c:4653!
>  Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
>  CPU: 1 PID: 2357 Comm: xfs_io 6.7.0-rc2-00195-g0f5cc96c367f
>  Hardware name: linux,dummy-virt (DT)
>  pc : ext4_mb_use_inode_pa+0x148/0x208
>  lr : ext4_mb_use_inode_pa+0x98/0x208
>  Call trace:
>   ext4_mb_use_inode_pa+0x148/0x208
>   ext4_mb_new_inode_pa+0x240/0x4a8
>   ext4_mb_use_best_found+0x1d4/0x208
>   ext4_mb_try_best_found+0xc8/0x110
>   ext4_mb_regular_allocator+0x11c/0xf48
>   ext4_mb_new_blocks+0x790/0xaa8
>   ext4_ext_map_blocks+0x7cc/0xd20
>   ext4_map_blocks+0x170/0x600
>   ext4_iomap_begin+0x1c0/0x348
> =========================================================
> 
> Here is a calculation when adjusting ac_b_ex in ext4_mb_new_inode_pa():
> 
> 	ex.fe_logical = orig_goal_end - EXT4_C2B(sbi, ex.fe_len);
> 	if (ac->ac_o_ex.fe_logical >= ex.fe_logical)
> 		goto adjust_bex;
> 
> The problem is that when orig_goal_end is subtracted from ac_b_ex.fe_len
> it is still greater than EXT_MAX_BLOCKS, which causes ex.fe_logical to
> overflow to a very small value, which ultimately triggers a BUG_ON in
> ext4_mb_new_inode_pa() because pa->pa_free < len.
> 
> The last logical block of an actual write request does not exceed
> EXT_MAX_BLOCKS, so in ext4_mb_normalize_request() also avoids normalizing
> the last logical block to exceed EXT_MAX_BLOCKS to avoid the above issue.
> 
> The test case in [Link] can reproduce the above issue with 64k block size.
> 
> Link: https://patchwork.kernel.org/project/fstests/list/?series=804003
> Cc: stable@xxxxxxxxxx # 6.4
> Fixes: 93cdf49f6eca ("ext4: Fix best extent lstart adjustment logic in ext4_mb_new_inode_pa()")
> Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>

Yeah, good catch. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  fs/ext4/mballoc.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 454d5612641e..d72b5e3c92ec 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -4478,6 +4478,10 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
>  	start = max(start, rounddown(ac->ac_o_ex.fe_logical,
>  			(ext4_lblk_t)EXT4_BLOCKS_PER_GROUP(ac->ac_sb)));
>  
> +	/* avoid unnecessary preallocation that may trigger assertions */
> +	if (start + size > EXT_MAX_BLOCKS)
> +		size = EXT_MAX_BLOCKS - start;
> +
>  	/* don't cover already allocated blocks in selected range */
>  	if (ar->pleft && start <= ar->lleft) {
>  		size -= ar->lleft + 1 - start;
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR