linux-kernel - Re: [PATCH 1/6] ext4: Add statx support for atomic writes

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Message-ID: <87y12cmr5o.fsf@gmail.com>
Date: Fri, 25 Oct 2024 15:38:03 +0530
From: Ritesh Harjani (IBM) <ritesh.list@...il.com>
To: John Garry <john.g.garry@...cle.com>, linux-ext4@...r.kernel.org
Cc: Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>, "Darrick J . Wong" <djwong@...nel.org>, Christoph Hellwig <hch@...radead.org>, Ojaswin Mujoo <ojaswin@...ux.ibm.com>, Dave Chinner <david@...morbit.com>, linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/6] ext4: Add statx support for atomic writes

John Garry <john.g.garry@...cle.com> writes:

> On 25/10/2024 04:45, Ritesh Harjani (IBM) wrote:
>> This patch adds base support for atomic writes via statx getattr.
>> On bs < ps systems, we can create FS with say bs of 16k. That means
>> both atomic write min and max unit can be set to 16k for supporting
>> atomic writes.
>> 
>> Later patches adds support for bigalloc as well so that ext4 can also
>> support doing atomic writes for bs = ps systems.
>> 
>> Co-developed-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
>> Signed-off-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
>> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>
>> ---
>>   fs/ext4/ext4.h  |  7 ++++++-
>>   fs/ext4/inode.c | 14 ++++++++++++++
>>   fs/ext4/super.c | 32 ++++++++++++++++++++++++++++++++
>>   3 files changed, 52 insertions(+), 1 deletion(-)
>> 
>> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
>> index 44b0d418143c..a41e56c2c628 100644
>> --- a/fs/ext4/ext4.h
>> +++ b/fs/ext4/ext4.h
>> @@ -1729,6 +1729,10 @@ struct ext4_sb_info {
>>   	 */
>>   	struct work_struct s_sb_upd_work;
>>   
>> +	/* Atomic write unit values */
>> +	unsigned int fs_awu_min;
>> +	unsigned int fs_awu_max;
>> +
>>   	/* Ext4 fast commit sub transaction ID */
>>   	atomic_t s_fc_subtid;
>>   
>> @@ -1820,7 +1824,8 @@ static inline int ext4_valid_inum(struct super_block *sb, unsigned long ino)
>>    */
>>   enum {
>>   	EXT4_MF_MNTDIR_SAMPLED,
>> -	EXT4_MF_FC_INELIGIBLE	/* Fast commit ineligible */
>> +	EXT4_MF_FC_INELIGIBLE,	/* Fast commit ineligible */
>> +	EXT4_MF_ATOMIC_WRITE	/* Supports atomic write */
>
> Does this flag really buy us much?
>

I felt it is cleaner this way than comparing non-zero values of
fs_awu_min and fs_awu_max.

Now that you pointed at it - Maybe a question for others who might have
the history of which one to use when - or do we think there is a scope
of merging the two into just one as a later cleanup?

I know that s_mount_flags was added for fastcommit and it needed the
state manipulations to be done in atomic way. Similarly s_ext4_flags
also was renamed from s_resize_flags for more general purpose use. Both
of these looks like could be merged isn't it?

>>   };
>>   
>>   static inline void ext4_set_mount_flag(struct super_block *sb, int bit)
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index 54bdd4884fe6..897c028d5bc9 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -5578,6 +5578,20 @@ int ext4_getattr(struct mnt_idmap *idmap, const struct path *path,
>>   		}
>>   	}
>>   
>> +	if (S_ISREG(inode->i_mode) && (request_mask & STATX_WRITE_ATOMIC)) {
>> +		struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
>> +		unsigned int awu_min, awu_max;
>> +
>> +		if (ext4_test_mount_flag(inode->i_sb, EXT4_MF_ATOMIC_WRITE)) {
>
> I'd use ext4_inode_can_atomicwrite() here, similar to what is done for xfs
>

Sure since it is inode operation, we can check against ext4_inode_can_atomicwrite().

>> +			awu_min = sbi->fs_awu_min;
>> +			awu_max = sbi->fs_awu_max;
>> +		} else {
>> +			awu_min = awu_max = 0;
>> +		}
>> +
>> +		generic_fill_statx_atomic_writes(stat, awu_min, awu_max);
>> +	}
>> +
>>   	flags = ei->i_flags & EXT4_FL_USER_VISIBLE;
>>   	if (flags & EXT4_APPEND_FL)
>>   		stat->attributes |= STATX_ATTR_APPEND;
>> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
>> index 16a4ce704460..f5c075aff060 100644
>> --- a/fs/ext4/super.c
>> +++ b/fs/ext4/super.c
>> @@ -4425,6 +4425,37 @@ static int ext4_handle_clustersize(struct super_block *sb)
>>   	return 0;
>>   }
>>   
>> +/*