linux-kernel - Re: question about bd_inode hashing against device_add() // Re: [PATCH 03/11] block: call bdev_add later in device_add

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Message-ID: <7d0d8480-13a2-449f-a46d-d9b164d44089@linux.alibaba.com>
Date: Fri, 31 Oct 2025 17:54:10 +0800
From: Gao Xiang <hsiangkao@...ux.alibaba.com>
To: Christoph Hellwig <hch@....de>
Cc: Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
 Christian Brauner <brauner@...nel.org>, "Michael S. Tsirkin"
 <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
 "Martin K. Petersen" <martin.petersen@...cle.com>,
 Luis Chamberlain <mcgrof@...nel.org>, linux-block@...r.kernel.org,
 Joseph Qi <joseph.qi@...ux.alibaba.com>, guanghuifeng@...ux.alibaba.com,
 zongyong.wzy@...baba-inc.com, zyfjeff@...ux.alibaba.com,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>, Danilo Krummrich <dakr@...nel.org>,
 linux-kernel@...r.kernel.org
Subject: Re: question about bd_inode hashing against device_add() // Re:
 [PATCH 03/11] block: call bdev_add later in device_add_disk

On 2025/10/31 17:45, Christoph Hellwig wrote:
> On Fri, Oct 31, 2025 at 05:36:45PM +0800, Gao Xiang wrote:
>> Right, sorry yes, disk_uevent(KOBJ_ADD) is in the end.
>>
>>>   Do you see that earlier, or do you have
>>> code busy polling for a node?
>>
>> Personally I think it will break many userspace programs
>> (although I also don't think it's a correct expectation.)
> 
> We've had this behavior for a few years, and this is the first report
> I've seen.
> 
>> After recheck internally, the userspace program logic is:
>>    - stat /dev/vdX;
>>    - if exists, mount directly;
>>    - if non-exists, listen uevent disk_add instead.
>>
>> Previously, for devtmpfs blkdev files, such stat/mount
>> assumption is always valid.
> 
> That assumption doesn't seem wrong.

;-) I was thought UNIX mknod doesn't imply the device is
ready or valid in any case (but dev files in devtmpfs
might be an exception but I didn't find some formal words)...
so uevent is clearly a right way, but..

> But why does the device node
> get created earlier?  My assumption was that it would only be
> created by the KOBJ_ADD uevent.  Adding the device model maintainers
> as my little dig through the core drivers/base/ code doesn't find
> anything to the contrary, but maybe I don't fully understand it.

AFAIK, device_add() is used to trigger devtmpfs file
creation, and it can be observed if frequently
hotpluging device in the VM and mount.  Currently
I don't have time slot to build an easy reproducer,
but I think it's a real issue anyway.

Thanks,
Gao Xiang