linux-kernel - Re: [RFC PATCH] dm ioctl: fix erroneous EINVAL when signaled

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Message-ID: <011a15cb-1f9e-456e-9374-5bca382d4e81@redhat.com>
Date: Mon, 22 Jul 2024 11:52:14 +0200
From: Zdenek Kabelac <zkabelac@...hat.com>
To: Khazhy Kumykov <khazhy@...omium.org>,
 Mikulas Patocka <mpatocka@...hat.com>
Cc: Alasdair Kergon <agk@...hat.com>, Mike Snitzer <snitzer@...nel.org>,
 Joe Thornber <thornber@...hat.com>, Heinz Mauelshagen <heinzm@...hat.com>,
 dm-devel@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] dm ioctl: fix erroneous EINVAL when signaled

Dne 17. 07. 24 v 21:52 Khazhy Kumykov napsal(a):
> On Wed, Jul 17, 2024 at 12:45 PM Mikulas Patocka <mpatocka@...hat.com> wrote:
>> Hi
>>
>> I am wondering why does do_resume need to call dm_suspend at all. Does
>> anyone here remember why is this code path needed?
> In our case, we have a sequence with load_table followed by a resume,
> with no suspend first. The resume path suspends if needed, swaps
> tables, then resumes. Removing the suspend here would break existing
> userspace, I'd imagine. It seems like minimizing the suspended time
> would also be a nice benefit.

lvm2 maintainer POV

Automatic 'suspend' for resume is a kernel 'feature' that should not be 
normally used from the userspace. Userspace is supposed to call    'suspend'  
- handle error cases - eventually drop preloaded table and resume existing 
table that should work.

If userspace is using ONLY  'resume'  without calling suspend upfront - there 
are some unsolvable error cases.

So no -  'minimizing'  suspend time is NOT the main reason here. The only 
valid reason to use it is basically if you are  admin and you need to reload 
table for a device you are running from - in this case calling 'dmsetup 
suspend'  might leave your system in 'blocked' state since your rootfs will be 
'frozen/suspend' and you would have no chance to  call 'dmsetup resume'.

lvm2 app is locking itself in the RAM in this critical section so it can 
proceed with regular sequence:    'write metadata - preload DM - suspend DM  - 
commit metadata - resume DM'  which basicall all userland apps should be using.

Regards

Zdenek