Re: [PATCH 3/3] ext4: fix race between writepages and remount

[Baokun Li <libaokun1@huawei.com>]

On 2023/3/27 17:35, Jan Kara wrote:
> On Thu 23-03-23 22:18:53, Baokun Li wrote:
>> On 2023/3/23 19:44, Jan Kara wrote:
>>>> ---
>>>>    fs/ext4/ext4.h      |  3 ++-
>>>>    fs/ext4/ext4_jbd2.h |  9 +++++----
>>>>    fs/ext4/super.c     | 14 ++++++++++++++
>>>>    3 files changed, 21 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
>>>> index 08b29c289da4..f60967fa648f 100644
>>>> --- a/fs/ext4/ext4.h
>>>> +++ b/fs/ext4/ext4.h
>>>> @@ -1703,7 +1703,8 @@ struct ext4_sb_info {
>>>>    	/*
>>>>    	 * Barrier between writepages ops and changing any inode's JOURNAL_DATA
>>>> -	 * or EXTENTS flag.
>>>> +	 * or EXTENTS flag or between changing SHOULD_DIOREAD_NOLOCK flag on
>>>> +	 * remount and writepages ops.
>>>>    	 */
>>>>    	struct percpu_rw_semaphore s_writepages_rwsem;
>>>>    	struct dax_device *s_daxdev;
>>>> diff --git a/fs/ext4/ext4_jbd2.h b/fs/ext4/ext4_jbd2.h
>>>> index 0c77697d5e90..d82bfcdd56e5 100644
>>>> --- a/fs/ext4/ext4_jbd2.h
>>>> +++ b/fs/ext4/ext4_jbd2.h
>>>> @@ -488,6 +488,9 @@ static inline int ext4_free_data_revoke_credits(struct inode *inode, int blocks)
>>>>    	return blocks + 2*(EXT4_SB(inode->i_sb)->s_cluster_ratio - 1);
>>>>    }
>>>> +/* delalloc is a temporary fix to prevent generic/422 test failures*/
>>>> +#define EXT4_MOUNT_SHOULD_DIOREAD_NOLOCK (EXT4_MOUNT_DIOREAD_NOLOCK | \
>>>> +					  EXT4_MOUNT_DELALLOC)
>>>>    /*
>>>>     * This function controls whether or not we should try to go down the
>>>>     * dioread_nolock code paths, which makes it safe to avoid taking
>>>> @@ -499,7 +502,8 @@ static inline int ext4_free_data_revoke_credits(struct inode *inode, int blocks)
>>>>     */
>>>>    static inline int ext4_should_dioread_nolock(struct inode *inode)
>>>>    {
>>>> -	if (!test_opt(inode->i_sb, DIOREAD_NOLOCK))
>>>> +	if (test_opt(inode->i_sb, SHOULD_DIOREAD_NOLOCK) !=
>>>> +	    EXT4_MOUNT_SHOULD_DIOREAD_NOLOCK)
>>>>    		return 0;
>>>>    	if (!S_ISREG(inode->i_mode))
>>>>    		return 0;
>>>> @@ -507,9 +511,6 @@ static inline int ext4_should_dioread_nolock(struct inode *inode)
>>>>    		return 0;
>>>>    	if (ext4_should_journal_data(inode))
>>>>    		return 0;
>>>> -	/* temporary fix to prevent generic/422 test failures */
>>>> -	if (!test_opt(inode->i_sb, DELALLOC))
>>>> -		return 0;
>>>>    	return 1;
>>>>    }
>>> Is there a need for this SHOULD_DIOREAD_NOLOCK? When called from writeback
>>> we will be protected by s_writepages_rwsem anyway. When called from other
>>> places, we either decide to do dioread_nolock or don't but the situation
>>> can change at any instant so I don't see how unifying this check would
>>> help. And the new SHOULD_DIOREAD_NOLOCK somewhat obfuscates what's going
>>> on.
>> We're thinking that the mount-related flags in
>> ext4_should_dioread_nolock() might be modified, such as DELALLOC being
>> removed because generic/422 test failures were fixed in some other way,
>> resulting in some unnecessary locking during remount, or for whatever
>> reason a mount-related flag was added to ext4_should_dioread_nolock(),
>> and we didn't make a synchronization change in __ext4_remount() that
>> would cause the problem to recur.  So we added this flag to this function
>> (instead of in ext4.h), so that when we change the mount option in
>> ext4_should_dioread_nolock(), we directly change this flag, and we don't
>> have to consider making synchronization changes in __ext4_remount().
>>
>> We have checked where this function is called and there are two types of
>> calls to this function:
>> 1. One category is ext4_do_writepages() and mpage_map_one_extent(), which
>> are protected by s_writepages_rwsem, the location of the problem;
>> 2. The other type is in ext4_page_mkwrite(),
>> ext4_convert_inline_data_to_extent(), ext4_write_begin() to determine
>> whether to get the block using ext4_get_block_unwritten() or
>> ext4_get_block().
>>
>>      1) If we just started fetching written blocks, it looks like there is no
>> problem;
>>      2) If we start getting unwritten blocks, when DIOREAD_NOLOCK is cleared
>> by remount,
>>          we will convert the blocks to written in ext4_map_blocks(). The
>> data=ordered mode ensures that we don't see stale data.
> Yes. So do you agree that EXT4_MOUNT_SHOULD_DIOREAD_NOLOCK is not really
> needed?
>
> 								Honza
Yes, I totally agree!
If we unconditionally grabbed s_writepages_rwsem when remounting,
there would be no mount option synchronization problem, and the flag
would be completely unnecessary.

I will send a patch V2 with the changes suggested by you.
-- 
With Best Regards,
Baokun Li
.