Re: [PATCH] md/raid10: free r10_bio on write NOWAIT exits

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

Pre-existing issues:
- [High] The MD layer and RAID10 driver sleep in multiple paths while handling a `REQ_NOWAIT` bio, violating the non-blocking semantics of the block layer.
- [High] The driver unconditionally uses `GFP_NOIO` for mempool and bio allocations, which can sleep when memory is low, violating `REQ_NOWAIT` non-blocking semantics.
--

--- Patch [1]: [PATCH] md/raid10: free r10_bio on write NOWAIT exits ---
commit 2e30580bb5bb2ae342c9d924bf8c217f5b6c832f
Author: Guangshuo Li <[email protected]>

md/raid10: free r10_bio on write NOWAIT exits

This commit fixes a memory leak in raid10_write_request() where the r10_bio
structure was not returned to the mempool on two REQ_NOWAIT early exit
paths. It ensures free_r10bio() is called before returning false for these
NOWAIT cases.

> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 0a3cfdd3f5df8..d480344c5b8f3 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -1364,6 +1364,7 @@ static bool raid10_write_request(struct mddev *mddev, struct bio *bio,
>  		DEFINE_WAIT(w);
>  		/* Bail out if REQ_NOWAIT is set for the bio */
>  		if (bio->bi_opf & REQ_NOWAIT) {
> +			free_r10bio(r10_bio);
>  			bio_wouldblock_error(bio);
>  			return false;
>  		}

[ ... ]

> @@ -1397,6 +1398,7 @@ static bool raid10_write_request(struct mddev *mddev, struct bio *bio,
>  		md_wakeup_thread(mddev->thread);
>  		if (bio->bi_opf & REQ_NOWAIT) {
>  			allow_barrier(conf);
> +			free_r10bio(r10_bio);
>  			bio_wouldblock_error(bio);
>  			return false;
>  		}

[Severity: High]
This isn't a bug introduced by this patch, but are there other blocking
operations in the NOWAIT path that could still violate the non-blocking
semantics?

If a REQ_NOWAIT bio does not hit these early exits, it proceeds down to
wait_blocked_dev() later in raid10_write_request(), which does not check
for REQ_NOWAIT:

	raid10_find_phys(conf, r10_bio);
	wait_blocked_dev(mddev, r10_bio);

If a member device is blocked, wait_blocked_dev() drops the barrier and
sleeps in md_wait_for_blocked_rdev() (which has a 5-second timeout) and
then sleeps again in wait_barrier(conf, false).

Additionally, if the bio crosses a chunk boundary, it reacquires the
barrier without checking for REQ_NOWAIT:

	if (r10_bio->sectors < bio_sectors(bio)) {
		allow_barrier(conf);
		bio = bio_submit_split_bioset(bio, r10_bio->sectors,
					      &conf->bio_split);
		wait_barrier(conf, false);

Could these unconditional sleeps cause the thread submitting the async I/O
to stall, potentially exhausting io_uring worker threads?

[Severity: High]
This is also a pre-existing issue, but does the driver unconditionally use
GFP_NOIO for allocations when handling a REQ_NOWAIT bio?

Before raid10_write_request() is even called, __make_request() allocates
the primary r10_bio using GFP_NOIO:

	r10_bio = mempool_alloc(&conf->r10bio_pool, GFP_NOIO);

Later, raid10_write_one_disk() clones the bio unconditionally using
GFP_NOIO:

	mbio = bio_alloc_clone(rdev->bdev, bio, GFP_NOIO, &mddev->bio_set);

If the mempool is depleted and memory pressure requires direct reclaim,
won't these GFP_NOIO allocations sleep and stall the IO submission thread?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1