Re: [RFC PATCH v11.1 2/2] mm, pmem, xfs: Introduce MF_MEM_REMOVE for unbind

[Shiyang Ruan <ruansy.fnst@fujitsu.com>]

在 2023/4/25 23:18, Darrick J. Wong 写道:
> On Tue, Apr 25, 2023 at 03:23:15PM +0200, Jan Kara wrote:
>> On Tue 25-04-23 20:47:35, Shiyang Ruan wrote:
>>>
>>>
>>> 在 2023/4/20 20:09, Jan Kara 写道:
>>>> On Thu 20-04-23 10:07:39, Shiyang Ruan wrote:
>>>>> 在 2023/4/12 18:52, Shiyang Ruan 写道:
>>>>>> This is a RFC HOTFIX.
>>>>>>
>>>>>> This hotfix adds a exclusive forzen state to make sure any others won't
>>>>>> thaw the fs during xfs_dax_notify_failure():
>>>>>>
>>>>>>      #define SB_FREEZE_EXCLUSIVE	(SB_FREEZE_COMPLETE + 2)
>>>>>> Using +2 here is because Darrick's patch[0] is using +1.  So, should we
>>>>>> make these definitions global?
>>>>>>
>>>>>> Another thing I can't make up my mind is: when another freezer has freeze
>>>>>> the fs, should we wait unitl it finish, or print a warning in dmesg and
>>>>>> return -EBUSY?
>>>>>>
>>>>>> Since there are at least 2 places needs exclusive forzen state, I think
>>>>>> we can refactor helper functions of freeze/thaw for them.  e.g.
>>>>>>      int freeze_super_exclusive(struct super_block *sb, int frozen);
>>>>>>      int thaw_super_exclusive(struct super_block *sb, int frozen);
>>>>>>
>>>>>> [0] https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/commit/?h=repair-fscounters&id=c3a0d1de4d54ffb565dbc7092dfe1fb851940669
>>>>
>>>> I'm OK with the idea of new freeze state that does not allow userspace to
>>>> thaw the filesystem. But I don't really like the guts of filesystem
>>>> freezing being replicated inside XFS. It is bad enough that they are
>>>> replicated in [0], replicating them *once more* in another XFS file shows
>>>> we are definitely doing something wrong. And Luis will need yet another
>>>> incantation of the exlusive freeze for suspend-to-disk. So please guys get
>>>> together and reorganize the generic freezing code so that it supports
>>>> exclusive freeze (for in-kernel users) and works for your usecases instead
>>>> of replicating it inside XFS...
>>>
>>> I agree that too much replicating code is not good.  It's necessary to
>>> create a generic exclusive freeze/thaw for all users.  But for me, I don't
>>> have the confidence to do it well, because it requires good design and code
>>> changes will involve other filesystems.  It's diffcult.
>>>
>>> However, I hope to be able to make progress on this unbind feature. Thus, I
>>> tend to refactor a common helper function for xfs first, and update the code
>>> later when the generic freeze is done.
>>
>> I think Darrick was thinking about working on a proper generic interface.
>> So please coordinate with him.
> 
> I'll post a vfs generic kernelfreeze series later today.
> 
> One thing I haven't figured out yet is what's supposed to happen when
> PREREMOVE is called on a frozen filesystem.

call PREREMOVE when:
1. freezed by kernel:    we wait unitl kernel thaws -> not sure
2. freezed by userspace: we take over the control of freeze state:
      a. userspace can't thaw before PREREMOVE is done
      b. kernel keeps freeze state after PREREMOVE is done and before 
userspace thaws

Since the unbind interface doesn't return any other errcode except 
-ENODEV, the only thing I can think of to do is wait for the other one 
done?  If another one doesn't thaw after a long time waitting, we print 
a "waitting too long" warning in dmesg.  But I'm not sure if this is good.

> We don't want userspace to
> be able to thaw the fs while PREREMOVE is running, so I /guess/ that
> means we need some method for the kernel to take over a userspace
> freeze and then put it back when we're done?

As is designed by Luis, we can add sb->s_writers.frozen_by_user flag to 
distinguish whether current freeze state is initiated by kernel or 
userspace.  In his patch, userspace can take over kernel's freeze.  We 
just need to switch the order.


--
Thanks,
Ruan.

> 
> --D
> 
>> 								Honza
>>
>> -- 
>> Jan Kara <jack@suse.com>
>> SUSE Labs, CR