linux-kernel - [PATCH] blk-throttle: check policy bit in blk_throtl

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Message-ID: <tencent_E4C0C41415118E05F82833A1453A097DD10A@qq.com>
Date: Tue,  2 Sep 2025 20:39:24 +0800
From: gj.han@...mail.com
To: Jens Axboe <axboe@...nel.dk>,
	linux-block@...r.kernel.org (open list:BLOCK LAYER),
	linux-kernel@...r.kernel.org (open list)
Cc: hanguangjiang@...iang.com,
	fanggeng@...iang.com,
	yangchen11@...iang.com,
	liangjie@...iang.com
Subject: [PATCH] blk-throttle: check policy bit in blk_throtl_activated()

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>
---
 block/blk-throttle.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-throttle.h b/block/blk-throttle.h
index 3b27755bfbff..9ca43dc56eda 100644
--- a/block/blk-throttle.h
+++ b/block/blk-throttle.h
@@ -156,7 +156,7 @@ void blk_throtl_cancel_bios(struct gendisk *disk);
 
 static inline bool blk_throtl_activated(struct request_queue *q)
 {
-	return q->td != NULL;
+	return q->td != NULL && test_bit(blkcg_policy_throtl.plid, q->blkcg_pols);
 }
 
 static inline bool blk_should_throtl(struct bio *bio)
-- 
2.25.1