linux-kernel - Re: [PATCH v2 2/2] blk-mq: fix blk_mq_tags double free while nr

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Message-ID: <a9ebef3c-8e80-443d-9532-348aa93126a1@linux.ibm.com>
Date: Tue, 19 Aug 2025 18:15:04 +0530
From: Nilay Shroff <nilay@...ux.ibm.com>
To: Yu Kuai <yukuai1@...weicloud.com>, yukuai3@...wei.com, axboe@...nel.dk,
        bvanassche@....org, ming.lei@...hat.com, hare@...e.de
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, yangerkun@...wei.com, johnny.chenyi@...wei.com
Subject: Re: [PATCH v2 2/2] blk-mq: fix blk_mq_tags double free while
 nr_requests grown



On 8/19/25 6:59 AM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.com>
> 
> In the case user trigger tags grow by queue sysfs attribute nr_requests,
> hctx->sched_tags will be freed directly and replaced with a new
> allocated tags, see blk_mq_tag_update_depth().
> 
> The problem is that hctx->sched_tags is from elevator->et->tags, while
> et->tags is still the freed tags, hence later elevator exist will try to
nit: 's/exist/exit'

> free the tags again, causing kernel panic.
> 
> Fix this problem by replacing et->tags will new allocated tags as well.
nit: 's/will/with'

> 
> Noted there are still some long term problems that will require some
> refactor to be fixed thoroughly[1].
> 
> [1] https://lore.kernel.org/all/20250815080216.410665-1-yukuai1@huaweicloud.com/
> Fixes: f5a6604f7a44 ("block: fix lockdep warning caused by lock dependency in elv_iosched_store")
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>  block/blk-mq-tag.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index d880c50629d6..5cffa5668d0c 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -622,6 +622,7 @@ int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
>  			return -ENOMEM;
>  
>  		blk_mq_free_map_and_rqs(set, *tagsptr, hctx->queue_num);
> +		hctx->queue->elevator->et->tags[hctx->queue_num] = new;
>  		*tagsptr = new;
>  	} else {
>  		/*
Except the above minor nitpicking this change looks good to me:

Reviewed-by: Nilay Shroff<nilay@...ux.ibm.com>