linux-kernel - Re: [PATCH v3] blk-throttle: fix access race during throttle policy activation

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Message-ID: <d918a88b-5dff-43e4-8cf6-77d1b716124f@yukuai.org.cn>
Date: Sat, 6 Sep 2025 02:18:27 +0800
From: Yu Kuai <hailan@...uai.org.cn>
To: Han Guangjiang <gj.han@...mail.com>, yukuai1@...weicloud.com,
 Jens Axboe <axboe@...nel.dk>,
 "open list:BLOCK LAYER" <linux-block@...r.kernel.org>,
 open list <linux-kernel@...r.kernel.org>
Cc: hanguangjiang@...iang.com, fanggeng@...iang.com, yangchen11@...iang.com,
 liangjie@...iang.com
Subject: Re: [PATCH v3] blk-throttle: fix access race during throttle policy
 activation

在 2025/9/5 18:24, Han Guangjiang 写道:

> From: Han Guangjiang<hanguangjiang@...iang.com>
>
> On repeated cold boots we occasionally hit a NULL pointer crash in
> blk_should_throtl() when throttling is consulted before the throttle
> policy is fully enabled for the queue. Checking only q->td != NULL is
> insufficient during early initialization, so blkg_to_pd() for the
> throttle policy can still return NULL and blkg_to_tg() becomes NULL,
> which later gets dereferenced.
>
>   Unable to handle kernel NULL pointer dereference
>   at virtual address 0000000000000156
>   ...
>   pc : submit_bio_noacct+0x14c/0x4c8
>   lr : submit_bio_noacct+0x48/0x4c8
>   sp : ffff800087f0b690
>   x29: ffff800087f0b690 x28: 0000000000005f90 x27: ffff00068af393c0
>   x26: 0000000000080000 x25: 000000000002fbc0 x24: ffff000684ddcc70
>   x23: 0000000000000000 x22: 0000000000000000 x21: 0000000000000000
>   x20: 0000000000080000 x19: ffff000684ddcd08 x18: ffffffffffffffff
>   x17: 0000000000000000 x16: ffff80008132a550 x15: 0000ffff98020fff
>   x14: 0000000000000000 x13: 1fffe000d11d7021 x12: ffff000688eb810c
>   x11: ffff00077ec4bb80 x10: ffff000688dcb720 x9 : ffff80008068ef60
>   x8 : 00000a6fb8a86e85 x7 : 000000000000111e x6 : 0000000000000002
>   x5 : 0000000000000246 x4 : 0000000000015cff x3 : 0000000000394500
>   x2 : ffff000682e35e40 x1 : 0000000000364940 x0 : 000000000000001a
>   Call trace:
>    submit_bio_noacct+0x14c/0x4c8
>    verity_map+0x178/0x2c8
>    __map_bio+0x228/0x250
>    dm_submit_bio+0x1c4/0x678
>    __submit_bio+0x170/0x230
>    submit_bio_noacct_nocheck+0x16c/0x388
>    submit_bio_noacct+0x16c/0x4c8
>    submit_bio+0xb4/0x210
>    f2fs_submit_read_bio+0x4c/0xf0
>    f2fs_mpage_readpages+0x3b0/0x5f0
>    f2fs_readahead+0x90/0xe8
>
> Tighten blk_throtl_activated() to also require that the throttle policy
> bit is set on the queue:
>
>    return q->td != NULL &&
>           test_bit(blkcg_policy_throtl.plid, q->blkcg_pols);
>
> This prevents blk_should_throtl() from accessing throttle group state
> until policy data has been attached to blkgs.
>
> Fixes: a3166c51702b ("blk-throttle: delay initialization until configuration")
> Co-developed-by: Liang Jie<liangjie@...iang.com>
> Signed-off-by: Liang Jie<liangjie@...iang.com>
> Signed-off-by: Han Guangjiang<hanguangjiang@...iang.com>
> ---
> v2:
>   - remove the comment about freeze queue in blk_should_throtl()
>   - Retitle: "blk-throttle: fix access race during throttle policy activation"
> v3:
>   - move blkcg_policy_enabled() to blk-cgroup.h
>   - use blkcg_policy_enabled() instead in blk_throtl_activated()
>   - remove the comment about freeze queue in blk_throtl_init()
>   - add some comments to decribe the throttle init synchronization
>   - Link to v2:https://lore.kernel.org/lkml/tencent_2B678DA920124B08854638A6BE68746CCC05@qq.com/
> ---
>   block/blk-cgroup.c   |  6 ------
>   block/blk-cgroup.h   |  6 ++++++
>   block/blk-throttle.c |  6 +-----
>   block/blk-throttle.h | 18 +++++++++++-------
>   4 files changed, 18 insertions(+), 18 deletions(-)

LGTM
Reviewed-by: Yu Kuai <yukuai3@...wei.com>