Re: [PATCH v2] loop: defer the queue limits clear to a workqueue

[Tao Cui]

Hi. Bart, 

在 2026/9/2 14:34, Tao Cui 写道:
> From: Tao Cui <cuitao@xxxxxxxxxx>
> 
> loop_clear_limits() calls queue_limits_commit_update() directly from
> the loop workqueue that processes the request.  That does a
> non-atomic struct assignment to q->limits without freezing the queue,
> which races with lockless readers of q->limits on other CPUs - bio
> splitting reads max_hw_sectors, the discard path reads
> max_hw_discard_sectors - and can let them observe torn values.  The
> trigger is a discard or write-zeroes request on a loop device whose
> backing file does not support the corresponding fallocate operation.
> 
> The code already has an XXX comment saying this should move to a
> workqueue.  Do that: schedule a work item on the system workqueue,
> where it is safe to freeze the queue and update the limits using
> queue_limits_commit_update_frozen().  Accumulate pending modes in
> lo->clear_limits_mode so that failures between scheduling and
> execution of the work item are not lost, reset the accumulated
> modes when a new backing file is assigned, and cancel the work
> item before the device is freed.
> 
> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
> 
> ---
> Changes since v1:
> 
> - Make clear_limits_mode atomic_t: loop workers can hit the |=
>   concurrently.
> 
> - Reset clear_limits_mode when assigning a new backing file, so
>   stale modes do not clear limits of the new file.
> 
> - Cancel the work item from loop_remove() before del_gendisk():
>   the queue can already be in RCU-delayed freeing when
>   lo_free_disk() cancels it.
> 
> Tested on x86-64 (qemu, vfat-backed loop device): 30s discard and
> reconfigure loop exercises the clear 577 times, no torn sysfs reads,
> no difference against the unpatched kernel.
> 

thanks for the Reviewed-by on v1.

v2 makes clear_limits_mode atomic, resets it on rebind, and
cancels the work item from loop_remove().  The code changed, so
could you take another look when you have time?

Thanks,
Tao

> Link: https://lore.kernel.org/r/20260828072004.273519-1-cui.tao@xxxxxxxxx/
> ---
>  drivers/block/loop.c | 29 ++++++++++++++++++++---------
>  1 file changed, 20 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 6f12976035b0..368ea857895c 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -67,6 +67,8 @@ struct loop_device {
>  	struct list_head        rootcg_cmd_list;
>  	struct list_head        idle_worker_list;
>  	struct rb_root          worker_tree;
> +	struct work_struct      clear_limits_work;
> +	atomic_t		clear_limits_mode;
>  	struct timer_list       timer;
>  	bool			sysfs_inited;
>  
> @@ -222,9 +224,12 @@ static void loop_set_size(struct loop_device *lo, loff_t size)
>  		kobject_uevent(&disk_to_dev(lo->lo_disk)->kobj, KOBJ_CHANGE);
>  }
>  
> -static void loop_clear_limits(struct loop_device *lo, int mode)
> +static void loop_clear_limits_workfn(struct work_struct *work)
>  {
> +	struct loop_device *lo =
> +		container_of(work, struct loop_device, clear_limits_work);
>  	struct queue_limits lim = queue_limits_start_update(lo->lo_queue);
> +	int mode = atomic_xchg(&lo->clear_limits_mode, 0);
>  
>  	if (mode & FALLOC_FL_ZERO_RANGE)
>  		lim.max_write_zeroes_sectors = 0;
> @@ -234,14 +239,13 @@ static void loop_clear_limits(struct loop_device *lo, int mode)
>  		lim.discard_granularity = 0;
>  	}
>  
> -	/*
> -	 * XXX: this updates the queue limits without freezing the queue, which
> -	 * is against the locking protocol and dangerous.  But we can't just
> -	 * freeze the queue as we're inside the ->queue_rq method here.  So this
> -	 * should move out into a workqueue unless we get the file operations to
> -	 * advertise if they support specific fallocate operations.
> -	 */
> -	queue_limits_commit_update(lo->lo_queue, &lim);
> +	queue_limits_commit_update_frozen(lo->lo_queue, &lim);
> +}
> +
> +static void loop_clear_limits(struct loop_device *lo, int mode)
> +{
> +	atomic_or(mode, &lo->clear_limits_mode);
> +	schedule_work(&lo->clear_limits_work);
>  }
>  
>  static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
> @@ -516,6 +520,7 @@ static int loop_validate_file(struct file *file, struct block_device *bdev)
>  static void loop_assign_backing_file(struct loop_device *lo, struct file *file)
>  {
>  	lo->lo_backing_file = file;
> +	atomic_set(&lo->clear_limits_mode, 0);
>  	lo->old_gfp_mask = mapping_gfp_mask(file->f_mapping);
>  	mapping_set_gfp_mask(file->f_mapping,
>  			lo->old_gfp_mask & ~(__GFP_IO | __GFP_FS));
> @@ -1781,6 +1786,7 @@ static void lo_free_disk(struct gendisk *disk)
>  		destroy_workqueue(lo->workqueue);
>  	loop_free_idle_workers(lo, true);
>  	timer_shutdown_sync(&lo->timer);
> +	cancel_work_sync(&lo->clear_limits_work);
>  	mutex_destroy(&lo->lo_mutex);
>  	kfree(lo);
>  }
> @@ -2100,6 +2106,7 @@ static int loop_add(int i)
>  	spin_lock_init(&lo->lo_lock);
>  	spin_lock_init(&lo->lo_work_lock);
>  	INIT_WORK(&lo->rootcg_work, loop_rootcg_workfn);
> +	INIT_WORK(&lo->clear_limits_work, loop_clear_limits_workfn);
>  	INIT_LIST_HEAD(&lo->rootcg_cmd_list);
>  	disk->major		= LOOP_MAJOR;
>  	disk->first_minor	= i << part_shift;
> @@ -2138,6 +2145,10 @@ static int loop_add(int i)
>  
>  static void loop_remove(struct loop_device *lo)
>  {
> +	/* Cancel early: the queue may be in RCU-delayed freeing
> +	 * by the time lo_free_disk() runs. */
> +	cancel_work_sync(&lo->clear_limits_work);
> +
>  	/* Make this loop device unreachable from pathname. */
>  	del_gendisk(lo->lo_disk);
>  	blk_mq_free_tag_set(&lo->tag_set);