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

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Message-ID: <87o738m5yk.fsf@gmail.com>
Date: Fri, 25 Oct 2024 23:15:55 +0530
From: Ritesh Harjani (IBM) <ritesh.list@...il.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: John Garry <john.g.garry@...cle.com>, linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>, 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

"Darrick J. Wong" <djwong@...nel.org> writes:

> On Fri, Oct 25, 2024 at 03:38:03PM +0530, Ritesh Harjani wrote:
>> 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.
>
> What does it mean when MF_ATOMIC_WRITE is set and fs_awu_* are zero?
> The awu values don't change at runtime, so I think you can save yourself
> an atomic test by checking (non-atomically) for awu_min>0.

Sure. I agree with the reasoning that we can just check for awu_min > 0.
I can write an inline helper for this.

>
> (I don't know anything about the flags, those came after my time iirc.)
>

Thanks for the review :) 

-ritesh

> --D
>
>> 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;
>> >>   }
>> >>   
>> >> +/*
>>