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Subject: Re: [f2fs-dev] [PATCH] Revert "f2fs: give a warning only for readonly
 partition"
To:     Jaegeuk Kim <jaegeuk@kernel.org>
CC:     <linux-kernel@vger.kernel.org>,
        <linux-f2fs-devel@lists.sourceforge.net>
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From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <84688aac-75da-1226-df4d-47ac97087c51@huawei.com>
Date:   Fri, 26 Mar 2021 09:34:22 +0800
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On 2021/3/26 9:19, Jaegeuk Kim wrote:
> On 03/26, Chao Yu wrote:
>> On 2021/3/25 9:59, Chao Yu wrote:
>>> On 2021/3/25 6:44, Jaegeuk Kim wrote:
>>>> On 03/24, Chao Yu wrote:
>>>>> On 2021/3/24 12:22, Jaegeuk Kim wrote:
>>>>>> On 03/24, Chao Yu wrote:
>>>>>>> On 2021/3/24 2:39, Jaegeuk Kim wrote:
>>>>>>>> On 03/23, Chao Yu wrote:
>>>>>>>>> This reverts commit 938a184265d75ea474f1c6fe1da96a5196163789.
>>>>>>>>>
>>>>>>>>> Because that commit fails generic/050 testcase which expect failure
>>>>>>>>> during mount a recoverable readonly partition.
>>>>>>>>
>>>>>>>> I think we need to change generic/050, since f2fs can recover this partition,
>>>>>>>
>>>>>>> Well, not sure we can change that testcase, since it restricts all generic
>>>>>>> filesystems behavior. At least, ext4's behavior makes sense to me:
>>>>>>>
>>>>>>> 	journal_dev_ro = bdev_read_only(journal->j_dev);
>>>>>>> 	really_read_only = bdev_read_only(sb->s_bdev) | journal_dev_ro;
>>>>>>>
>>>>>>> 	if (journal_dev_ro && !sb_rdonly(sb)) {
>>>>>>> 		ext4_msg(sb, KERN_ERR,
>>>>>>> 			 "journal device read-only, try mounting with '-o ro'");
>>>>>>> 		err = -EROFS;
>>>>>>> 		goto err_out;
>>>>>>> 	}
>>>>>>>
>>>>>>> 	if (ext4_has_feature_journal_needs_recovery(sb)) {
>>>>>>> 		if (sb_rdonly(sb)) {
>>>>>>> 			ext4_msg(sb, KERN_INFO, "INFO: recovery "
>>>>>>> 					"required on readonly filesystem");
>>>>>>> 			if (really_read_only) {
>>>>>>> 				ext4_msg(sb, KERN_ERR, "write access "
>>>>>>> 					"unavailable, cannot proceed "
>>>>>>> 					"(try mounting with noload)");
>>>>>>> 				err = -EROFS;
>>>>>>> 				goto err_out;
>>>>>>> 			}
>>>>>>> 			ext4_msg(sb, KERN_INFO, "write access will "
>>>>>>> 			       "be enabled during recovery");
>>>>>>> 		}
>>>>>>> 	}
>>>>>>>
>>>>>>>> even though using it as readonly. And, valid checkpoint can allow for user to
>>>>>>>> read all the data without problem.
>>>>>>>
>>>>>>>>>       		if (f2fs_hw_is_readonly(sbi)) {
>>>>>>>
>>>>>>> Since device is readonly now, all write to the device will fail, checkpoint can
>>>>>>> not persist recovered data, after page cache is expired, user can see stale data.
>>>>>>
>>>>>> My point is, after mount with ro, there'll be no data write which preserves the
>>>>>> current status. So, in the next time, we can recover fsync'ed data later, if
>>>>>> user succeeds to mount as rw. Another point is, with the current checkpoint, we
>>>>>> should not have any corrupted metadata. So, why not giving a chance to show what
>>>>>> data remained to user? I think this can be doable only with CoW filesystems.
>>>>>
>>>>> I guess we're talking about the different things...
>>>>>
>>>>> Let me declare two different readonly status:
>>>>>
>>>>> 1. filesystem readonly: file system is mount with ro mount option, and
>>>>> app from userspace can not modify any thing of filesystem, but filesystem
>>>>> itself can modify data on device since device may be writable.
>>>>>
>>>>> 2. device readonly: device is set to readonly status via 'blockdev --setro'
>>>>> command, and then filesystem should never issue any write IO to the device.
>>>>>
>>>>> So, what I mean is, *when device is readonly*, rather than f2fs mountpoint
>>>>> is readonly (f2fs_hw_is_readonly() returns true as below code, instead of
>>>>> f2fs_readonly() returns true), in this condition, we should not issue any
>>>>> write IO to device anyway, because, AFAIK, write IO will fail due to
>>>>> bio_check_ro() check.
>>>>
>>>> In that case, mount(2) will try readonly, no?
>>>
>>> Yes, if device is readonly, mount (2) can not mount/remount device to rw
>>> mountpoint.
>>
>> Any other concern about this patch?
> 
> Indeed we're talking about different things. :)
> 
> This case is mount(ro) with device(ro) having some data to recover.
> My point is why not giving a chance to mount(ro) to show the current data
> covered by a valid checkpoint. This doesn't change anything in the disk,
Got your idea.

IMO, it has potential issue in above condition:

 >>>>>>> Since device is readonly now, all write to the device will fail, checkpoint can
 >>>>>>> not persist recovered data, after page cache is expired, user can see stale data.

e.g.

Recovery writes one inode and then triggers a checkpoint, all writes fail
due to device is readonly, once inode cache is reclaimed by vm, user will see
old inode when reloading it, or even see corrupted fs if partial meta inode's
cache is expired.

Thoughts?

Thanks,

> and in the next time, it allows mount(rw|ro) with device(rw) to recover
> the data seamlessly.
> 
>>
>> Thanks,
>>
>>>
>>> Thanks,
>>>
>>>>
>>>> # blockdev --setro /dev/vdb
>>>> # mount -t f2fs /dev/vdb /mnt/test/
>>>> mount: /mnt/test: WARNING: source write-protected, mounted read-only.
>>>>
>>>>>
>>>>>     		if (f2fs_hw_is_readonly(sbi)) {
>>>>> -			if (!is_set_ckpt_flags(sbi, CP_UMOUNT_FLAG)) {
>>>>> -				err = -EROFS;
>>>>> +			if (!is_set_ckpt_flags(sbi, CP_UMOUNT_FLAG))
>>>>>     				f2fs_err(sbi, "Need to recover fsync data, but write access unavailable");
>>>>> -				goto free_meta;
>>>>> -			}
>>>>> -			f2fs_info(sbi, "write access unavailable, skipping recovery");
>>>>> +			else
>>>>> +				f2fs_info(sbi, "write access unavailable, skipping recovery");
>>>>>     			goto reset_checkpoint;
>>>>>     		}
>>>>>
>>>>> For the case of filesystem is readonly and device is writable, it's fine
>>>>> to do recovery in order to let user to see fsynced data.
>>>>>
>>>>> Thanks,
>>>>>
>>>>>>
>>>>>>>
>>>>>>> Am I missing something?
>>>>>>>
>>>>>>> Thanks,
>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>> Fixes: 938a184265d7 ("f2fs: give a warning only for readonly partition")
>>>>>>>>> Signed-off-by: Chao Yu <yuchao0@huawei.com>
>>>>>>>>> ---
>>>>>>>>>       fs/f2fs/super.c | 8 +++++---
>>>>>>>>>       1 file changed, 5 insertions(+), 3 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
>>>>>>>>> index b48281642e98..2b78ee11f093 100644
>>>>>>>>> --- a/fs/f2fs/super.c
>>>>>>>>> +++ b/fs/f2fs/super.c
>>>>>>>>> @@ -3952,10 +3952,12 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
>>>>>>>>>       		 * previous checkpoint was not done by clean system shutdown.
>>>>>>>>>       		 */
>>>>>>>>>       		if (f2fs_hw_is_readonly(sbi)) {
>>>>>>>>> -			if (!is_set_ckpt_flags(sbi, CP_UMOUNT_FLAG))
>>>>>>>>> +			if (!is_set_ckpt_flags(sbi, CP_UMOUNT_FLAG)) {
>>>>>>>>> +				err = -EROFS;
>>>>>>>>>       				f2fs_err(sbi, "Need to recover fsync data, but write access unavailable");
>>>>>>>>> -			else
>>>>>>>>> -				f2fs_info(sbi, "write access unavailable, skipping recovery");
>>>>>>>>> +				goto free_meta;
>>>>>>>>> +			}
>>>>>>>>> +			f2fs_info(sbi, "write access unavailable, skipping recovery");
>>>>>>>>>       			goto reset_checkpoint;
>>>>>>>>>       		}
>>>>>>>>> -- 
>>>>>>>>> 2.29.2
>>>>>>>> .
>>>>>>>>
>>>>>> .
>>>>>>
>>>> .
>>>>
>>>
>>>
>>> _______________________________________________
>>> Linux-f2fs-devel mailing list
>>> Linux-f2fs-devel@lists.sourceforge.net
>>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
>>> .
>>>
> .
>