Re: [PATCH] block: elevator: avoid to load iosched module from this disk

[Ming Lei <ming.lei@redhat.com>]

On Sat, Sep 07, 2024 at 06:04:59PM +0900, Damien Le Moal wrote:
> On 9/7/24 16:58, Ming Lei wrote:
> > On Sat, Sep 07, 2024 at 08:35:22AM +0100, Richard W.M. Jones wrote:
> >> On Sat, Sep 07, 2024 at 09:43:31AM +0800, Ming Lei wrote:
> >>> When switching io scheduler via sysfs, 'request_module' may be called
> >>> if the specified scheduler doesn't exist.
> >>>
> >>> This was has deadlock risk because the module may be stored on FS behind
> >>> our disk since request queue is frozen before switching its elevator.
> >>>
> >>> Fix it by returning -EDEADLK in case that the disk is claimed, which
> >>> can be thought as one signal that the disk is mounted.
> >>>
> >>> Some distributions(Fedora) simulates the original kernel command line of
> >>> 'elevator=foo' via 'echo foo > /sys/block/$DISK/queue/scheduler', and boot
> >>> hang is triggered.
> >>>
> >>> Cc: Richard Jones <rjones@redhat.com>
> >>> Cc: Jeff Moyer <jmoyer@redhat.com>
> >>> Cc: Jiri Jaburek <jjaburek@redhat.com>
> >>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> >>
> >> I'd suggest also:
> >>
> >> Bug: https://bugzilla.kernel.org/show_bug.cgi?id=219166
> >> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> >> Reported-by: Jiri Jaburek <jjaburek@redhat.com>
> >> Tested-by: Richard W.M. Jones <rjones@redhat.com>
> >>
> >> So I have tested this patch and it does fix the issue, at the possible
> >> cost that now setting the scheduler can fail:
> >>
> >>   + for f in /sys/block/{h,s,ub,v}d*/queue/scheduler
> >>   + echo noop
> >>   /init: line 109: echo: write error: Resource deadlock avoided
> >>
> >> (I know I'm setting it to an impossible value here, but this could
> >> also happen when setting it to a valid one.)
> > 
> > Actually in most of dist, io-schedulers are built-in, so request_module
> > is just a nop, but meta IO must be started.
> > 
> >>
> >> Since almost no one checks the result of 'echo foo > /sys/...'  that
> >> would probably mean that sometimes a desired setting is silently not
> >> set.
> > 
> > As I mentioned, io-schedulers are built-in for most of dist, so
> > request_module isn't called in case of one valid io-sched.
> > 
> >>
> >> Also I bisected this bug yesterday and found it was caused by (or,
> >> more likely, exposed by):
> >>
> >>   commit af2814149883e2c1851866ea2afcd8eadc040f79
> >>   Author: Christoph Hellwig <hch@lst.de>
> >>   Date:   Mon Jun 17 08:04:38 2024 +0200
> >>
> >>     block: freeze the queue in queue_attr_store
> >>     
> >>     queue_attr_store updates attributes used to control generating I/O, and
> >>     can cause malformed bios if changed with I/O in flight.  Freeze the queue
> >>     in common code instead of adding it to almost every attribute.
> >>
> >> Reverting this commit on top of git head also fixes the problem.
> >>
> >> Why did this commit expose the problem?
> > 
> > That is really the 1st bad commit which moves queue freezing before
> > calling request_module(), originally we won't freeze queue until
> > we have to do it.
> > 
> > Another candidate fix is to revert it, or at least not do it
> > for storing elevator attribute.
> 
> I do not think that reverting is acceptable. Rather, a proper fix would simply

Right, I remember that the freezing starts to cover update of
max_sectors_kb.

> be to do the request_module() before freezing the queue.
> Something like below should work (totally untested and that may be overkill).
> 
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 60116d13cb80..aef87f6b4a8a 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -23,6 +23,7 @@
>  struct queue_sysfs_entry {
>         struct attribute attr;
>         ssize_t (*show)(struct gendisk *disk, char *page);
> +       int (*pre_store)(struct gendisk *disk, const char *page, size_t count);

It seems over-kill to add one new callback, and another way is just to
not freeze queue for storing elevator.

But if other attribute update needs to not freeze queue, 'pre_store'
looks one reasonable solution.

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 60116d13cb80..c418edf66f0c 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -666,15 +666,24 @@ queue_attr_store(struct kobject *kobj, struct attribute *attr,
 	struct gendisk *disk = container_of(kobj, struct gendisk, queue_kobj);
 	struct request_queue *q = disk->queue;
 	ssize_t res;
+	bool need_freeze;
 
 	if (!entry->store)
 		return -EIO;
 
-	blk_mq_freeze_queue(q);
+	/*
+	 * storing scheduler freezes queue in its way, especially
+	 * loading scheduler module can't be done when queue is frozen
+	 */
+	need_freeze = (entry->store == elv_iosched_store);
+
+	if (need_freeze)
+		blk_mq_freeze_queue(q);
 	mutex_lock(&q->sysfs_lock);
 	res = entry->store(disk, page, length);
 	mutex_unlock(&q->sysfs_lock);
-	blk_mq_unfreeze_queue(q);
+	if (need_freeze)
+		blk_mq_unfreeze_queue(q);
 	return res;
 }
 

Thanks,
Ming