Re: [PATCH v3 17/21] md/raid10: split reshape bios before bitmap accounting

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] Missing `md_write_end()` when `mddev_bio_split_at_reshape_offset()` fails on a write bio, leaking `mddev->writes_pending` reference count.
- [High] Mempool exhaustion deadlock due to double bio allocation from `conf->bio_split` without a rescuer thread.
--

--- Patch [17]: [PATCH v3 17/21] md/raid10: split reshape bios before bitmap accounting ---
commit f435d38ffb898e662e1e9b6225ad90d266d26363
Author: Yu Kuai <[email protected]>

md/raid10: split reshape bios before bitmap accounting
    
This patch updates raid10 to use the shared mddev_bio_split_at_reshape_offset()
helper. This ensures that RAID10 submits only one-side bios to llbitmap during
reshape operations.

> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index e50e4adee389f..fd364a2e2fd98 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -1911,6 +1911,12 @@ static bool raid10_make_request(struct mddev *mddev, struct bio *bio)
>  		sectors = chunk_sects -
>  			(bio->bi_iter.bi_sector &
>  			 (chunk_sects - 1));
> +
> +	bio = mddev_bio_split_at_reshape_offset(mddev, bio, &sectors,
> +						&conf->bio_split);

[Severity: High]
Could this lead to a mempool exhaustion deadlock if bad blocks are present?

This splits the bio using conf->bio_split, and passes the split bio down
to __make_request(). If there are bad blocks that restrict the sector count
further, the lower level routines perform a second split using the exact
same bio_set:

drivers/md/raid10.c:raid10_write_request() {
    ...
    bio = bio_submit_split_bioset(bio, r10_bio->sectors,
                                  &conf->bio_split);
    ...
}

Since conf->bio_split lacks a rescuer thread, would two concurrent I/Os
that both cross a chunk boundary and hit a bad block exhaust the pool and
stall indefinitely?

> +	if (!bio)
> +		return true;

[Severity: High]
Does this early return leak the mddev->writes_pending reference?

Earlier in raid10_make_request(), we call md_write_start() which increments
the writes_pending count for write bios:

drivers/md/raid10.c:raid10_make_request() {
    ...
    md_write_start(mddev, bio);
    ...
}

If the bio is an atomic write that cannot be split,
mddev_bio_split_at_reshape_offset() returns NULL. When we return true here,
we bypass the md_write_end() cleanup at the end of the function, which
might hang the array when it is later suspended or stopped.

> +
>  	if (!__make_request(mddev, bio, sectors))
>  		md_write_end(mddev);

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=17