Re: [PATCH -next] block: fix blktrace debugfs entries leak

[Yu Kuai]

Hi, Christoph

在 2023/05/12 15:14, Yu Kuai 写道:
> Hi,
>
> 在 2023/05/11 23:28, Christoph Hellwig 写道:
> > On Thu, May 11, 2023 at 02:56:33PM +0800, Yu Kuai wrote:
> > > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> > >
> > > Commit 99d055b4fd4b ("block: remove per-disk debugfs files in
> > > blk_unregister_queue") moves blk_trace_shutdown() from
> > > blk_release_queue() to blk_unregister_queue(), this is safe if blktrace
> > > is created through sysfs, however, there are some regression in corner
> > > cases:
> > >
> > > 1) for scsi, passthrough io can still be issued after del_gendisk, and
> > >     blktrace debugfs entries will be removed immediately after
> > >     del_gendisk(), therefor passthrough io can't be tracked and blktrace
> > >     will complain:
> > >
> > >     failed read of /sys/kernel/debug/block/sdb/trace0: 5/Input/output 
> > > error
> >
> > But that is the right thing.  The only thing that has a name is the
> > gendisk and it is gone at this point.  Leaking the debugfs entries
> > that are named after, and ultimatively associated with the gendisk
> > (even if the code is still a bit confused about this) will create a lot
> > of trouble for us.
> >
> > > 2) blktrace can still be enabled after del_gendisk() through ioctl if 
> > > the
> > >     disk is opened before del_gendisk(), and if blktrace is not shutdown
> > >     through ioctl before closing the disk, debugfs entries will be
> > >     leaked.
> >
> > Yes.
> >
> > > It seems 1) is not important, while 2) needs to be fixed apparently.
> > >
> > > Fix this problem by shutdown blktrace in blk_free_queue(),
> > > disk_release() is not used because scsi sg support blktrace without
> > > gendisk, and this is safe because queue is not freed yet, and
> > > blk_trace_shutdown() is reentrant.
> >
> > I think disk_release is the right place for "normal" blktrace.  The
> > odd cdev based blktrace for /dev/sg will need separate handling.
> > To be honest I'm not even sure how /dev/sg based passthrough is
> > even supposed to work in practice, but I'll need to spend some more
> > time to familarize myself with it.
>
> I'm not sure how to specail hanlde /dev/sg* for now, however,
> If we don't care about blktrace for passthrough io after del_gendisk(),
> and /dev/sg* has separate handling, I think it's better just to check
> QUEUE_FLAG_REGISTERED in blk_trace_setup(), and don't enable blktrace
> in the first place.

Any suggestions about this problem? Should we use separate handling for
/dev/sd? Or just free blktrace in blk_free_queue().
> Thanks,
> Kuai
>
> .