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

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Date:   Fri, 12 May 2023 15:14:23 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Christoph Hellwig <hch@....de>, Yu Kuai <yukuai1@...weicloud.com>
Cc:     ming.lei@...hat.com, axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        yangerkun@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH -next] block: fix blktrace debugfs entries leak

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@...wei.com>
>>
>> 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.

Thanks,
Kuai