Re: [PATCH v5 07/29] md: avoid stale clone I/O accounting timestamps

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Permanent in-flight I/O counter leak when jiffies wraps to 0.
--

--- Patch [7]: [PATCH v5 07/29] md: avoid stale clone I/O accounting timestamps ---
commit 3831c24d41ef696f06d6f215b39197db739a0831
Author: Yu Kuai <yukuai@fygo.io>

md: avoid stale clone I/O accounting timestamps
    
This commit avoids stale clone I/O accounting timestamps by explicitly
setting start_time to 0 when queue I/O stats are disabled. This prevents
the completion path in md_end_clone_io() from using a stale non-zero value 
from a pooled bio and performing accounting that was never started.
    
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -9448,6 +9448,8 @@ static void md_clone_bio(struct mddev *mddev, struct bio **bio)
>  	md_io_clone->mddev = mddev;
>  	if (blk_queue_io_stat(bdev->bd_disk->queue))
>  		md_io_clone->start_time = bio_start_io_acct(*bio);
> +	else
> +		md_io_clone->start_time = 0;

[Severity: High]
This is a pre-existing issue, but does using 0 as a sentinel value here
create a permanent in-flight I/O counter leak when jiffies wraps to 0?

In bio_start_io_acct(), the returned value is the current jiffies. On 32-bit
systems, jiffies is initialized to -300*HZ and naturally wraps to 0 exactly
five minutes after boot. If a bio is cloned exactly at the tick where
jiffies is 0, start_time will be recorded as 0 even though accounting
was started.

When completion occurs in md_end_clone_io(), the check sees 0:

    if (md_io_clone->start_time)
        bio_end_io_acct(orig_bio, md_io_clone->start_time);

This evaluates to false, bypassing the end accounting and leaving the
in-flight counter permanently incremented. This can cause operations that
wait for a queue freeze, such as suspend or device removal, to hang
indefinitely.

Would it be safer to use a separate boolean flag to track if accounting was
started, rather than overloading start_time?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260802195038.164272-1-yukuai@kernel.org?part=7