Re: [PATCH v4 1/3] md/raid5: freeze and interrupt reshape when handling failed reshape

[Chen Cheng]

On 2026/10/16 12:25, Doug V Johnson wrote:
> Set the reshape freeze flag to prevent automatic reshape resumption
> and interrupt the current reshape with md_sync_error() so that a
> reshape always stops no matter how the failed reshape is triggered.
> 
> Previously, if there were no faulty or out of sync devices but there
> was an unrecoverable read, the reshape continued and was not cancelled
> after handle_failed_reshape(). This caused some data corruption as
> block writes were cancelled but the reshape nonetheless continued.
> 
> Prior to Chen's fix, the kernel would deadlock with a failed stripe
> read.  That case also caused some data corruption, so Chen's patch
> did not introduce corruption in this case.
> 
> An example test case showing how this happened before this fix is
> demonstrated by the following:
> 
> Simplified example (schematic logical sector numbers):
> 
>    1. Before reshape, destination D contains data for logical 20-27.
>       The new geometry will use D for logical 100-107.
> 
>    2. Inject bad-block metadata on enough members that the old source
>       stripe cannot be reconstructed. Logical sector 100 now returns
>       a read error; sectors 101-107 are still readable.
> 
>    3. Reshape tries to read the source block for logical 100-107.
>       The bad-block records prevent a successful read/reconstruction.
>       Failure cleanup cancels the destination write:
> 
>         Source: logical 100-107 --X--> destination D
>                                       still contains data for 20-27
> 
>    4. Without interruption, reshape continues and completes.
>       The new mapping now sends reads of logical 100-107 to D.
> 
>    5. Reading logical 100-107 succeeds, but returns data for 20-27.
>       Previously readable sectors 101-107 now return wrong data.
> 
> Code that injects bad blocks to demonstrate this problem before and
> after Chen's fix as well as after this patch can be found here:
> 
> Link: https://github.com/dougvj/md-raid-reshape-badblock-corruption-test
> 
> Fixes: 788e4139463f ("md/raid5: fix reshape deadlock while failed devices more than max degraded")
> Link: https://patch.msgid.link/20260624075824.2601110-1-chencheng@xxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Doug V Johnson <me@xxxxxxxxx>
> ---
>   drivers/md/raid5.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index b91545ce090d..86ea65559576 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -3773,6 +3773,10 @@ static void handle_failed_reshape(struct r5conf *conf, struct stripe_head *sh,
>   	bool was_expanding = test_and_clear_bit(STRIPE_EXPANDING, &sh->state);
>   	bool was_ready = test_and_clear_bit(STRIPE_EXPAND_READY, &sh->state);
>   
> +	/* Freeze reshaping to prevent data loss */
> +	set_bit(MD_RECOVERY_FROZEN, &conf->mddev->recovery);
> +	md_sync_error(conf->mddev);
> +
>   	if (was_expanding || was_ready) {
>   		atomic_dec(&conf->reshape_stripes);
>   		wake_up(&conf->wait_for_reshape);

Reviewed-by: Chen Cheng <chencheng@xxxxxxxxx>