Re: [PATCH v4 04/11] fs: Add initial atomic write support info to statx

[John Garry <john.g.garry@oracle.com>]

On 24/02/2024 18:46, Ritesh Harjani (IBM) wrote:
> John Garry <john.g.garry@oracle.com> writes:
> 
>> From: Prasad Singamsetty <prasad.singamsetty@oracle.com>
>>
>> Extend statx system call to return additional info for atomic write support
>> support for a file.
>>
>> Helper function generic_fill_statx_atomic_writes() can be used by FSes to
>> fill in the relevant statx fields.
>>
>> Signed-off-by: Prasad Singamsetty <prasad.singamsetty@oracle.com>
>> #jpg: relocate bdev support to another patch
> 
> ^^^ miss maybe?
>> Signed-off-by: John Garry <john.g.garry@oracle.com>
>> ---
>>   fs/stat.c                 | 34 ++++++++++++++++++++++++++++++++++
>>   include/linux/fs.h        |  3 +++
>>   include/linux/stat.h      |  3 +++
>>   include/uapi/linux/stat.h |  9 ++++++++-
>>   4 files changed, 48 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/stat.c b/fs/stat.c
>> index 77cdc69eb422..522787a4ab6a 100644
>> --- a/fs/stat.c
>> +++ b/fs/stat.c
>> @@ -89,6 +89,37 @@ void generic_fill_statx_attr(struct inode *inode, struct kstat *stat)
>>   }
>>   EXPORT_SYMBOL(generic_fill_statx_attr);
>>   
>> +/**
>> + * generic_fill_statx_atomic_writes - Fill in the atomic writes statx attributes
>> + * @stat:	Where to fill in the attribute flags
>> + * @unit_min:	Minimum supported atomic write length
> + * @unit_min:	Minimum supported atomic write length in bytes
> 
> 
>> + * @unit_max:	Maximum supported atomic write length
> + * @unit_max:	Maximum supported atomic write length in bytes
> 
> mentioning unit of the length might be useful here.

Yeah, I have already improved this as suggested.

> 
>> + *
>> + * Fill in the STATX{_ATTR}_WRITE_ATOMIC flags in the kstat structure from
>> + * atomic write unit_min and unit_max values.
>> + */
>> +void generic_fill_statx_atomic_writes(struct kstat *stat,
>> +				      unsigned int unit_min,
> 
> This (unit_min) can still go above in the same line.

ok

> 
>> +				      unsigned int unit_max)
>> +{
>> +	/* Confirm that the request type is known */
>> +	stat->result_mask |= STATX_WRITE_ATOMIC;
>> +
>> +	/* Confirm that the file attribute type is known */
>> +	stat->attributes_mask |= STATX_ATTR_WRITE_ATOMIC;
>> +
>> +	if (unit_min) {
>> +		stat->atomic_write_unit_min = unit_min;
>> +		stat->atomic_write_unit_max = unit_max;
>> +		/* Initially only allow 1x segment */
>> +		stat->atomic_write_segments_max = 1;
> 
> Please log info about this in commit message about where this limit came
> from?

ok

> Is it since we only support ubuf (which IIUC, only supports 1
> segment)? Later when we will add support for iovec, this limit can be
> lifted?

It's not that we only support ubuf, but rather we only support one 
segment and that gives a ubuf type iter.

This is all related to how can can guarantee a unit_max advertised to 
userspace can always be written atomically.

This is further mentioned in the block layer patch.

> 
>> +
>> +		/* Confirm atomic writes are actually supported */
>> +		stat->attributes |= STATX_ATTR_WRITE_ATOMIC;
>> +	}
>> +}
>> +EXPORT_SYMBOL(generic_fill_statx_atomic_writes);
>> +
>>   /**
>>    * vfs_getattr_nosec - getattr without security checks
>>    * @path: file to get attributes from
>> @@ -658,6 +689,9 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
>>   	tmp.stx_mnt_id = stat->mnt_id;
>>   	tmp.stx_dio_mem_align = stat->dio_mem_align;
>>   	tmp.stx_dio_offset_align = stat->dio_offset_align;
>> +	tmp.stx_atomic_write_unit_min = stat->atomic_write_unit_min;
>> +	tmp.stx_atomic_write_unit_max = stat->atomic_write_unit_max;
>> +	tmp.stx_atomic_write_segments_max = stat->atomic_write_segments_max;
>>   
>>   	return copy_to_user(buffer, &tmp, sizeof(tmp)) ? -EFAULT : 0;
>>   }
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index 7271640fd600..531140a7e27a 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -3167,6 +3167,9 @@ extern const struct inode_operations page_symlink_inode_operations;
>>   extern void kfree_link(void *);
>>   void generic_fillattr(struct mnt_idmap *, u32, struct inode *, struct kstat *);
>>   void generic_fill_statx_attr(struct inode *inode, struct kstat *stat);
>> +void generic_fill_statx_atomic_writes(struct kstat *stat,
>> +				      unsigned int unit_min,
>> +				      unsigned int unit_max);
> 
> We can make 80 col. width even with unit_min in the same first line as of *stat.

ok, I can check this.

> 
> 
>>   extern int vfs_getattr_nosec(const struct path *, struct kstat *, u32, unsigned int);
>>   extern int vfs_getattr(const struct path *, struct kstat *, u32, unsigned int);
>>   void __inode_add_bytes(struct inode *inode, loff_t bytes);
>> diff --git a/include/linux/stat.h b/include/linux/stat.h
>> index 52150570d37a..2c5e2b8c6559 100644
>> --- a/include/linux/stat.h
>> +++ b/include/linux/stat.h
>> @@ -53,6 +53,9 @@ struct kstat {
>>   	u32		dio_mem_align;
>>   	u32		dio_offset_align;
>>   	u64		change_cookie;
>> +	u32		atomic_write_unit_min;
>> +	u32		atomic_write_unit_max;
>> +	u32		atomic_write_segments_max;
>>   };
>>   
>>   /* These definitions are internal to the kernel for now. Mainly used by nfsd. */
>> diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
>> index 2f2ee82d5517..c0e8e10d1de6 100644
>> --- a/include/uapi/linux/stat.h
>> +++ b/include/uapi/linux/stat.h
>> @@ -127,7 +127,12 @@ struct statx {
>>   	__u32	stx_dio_mem_align;	/* Memory buffer alignment for direct I/O */
>>   	__u32	stx_dio_offset_align;	/* File offset alignment for direct I/O */
>>   	/* 0xa0 */
>> -	__u64	__spare3[12];	/* Spare space for future expansion */
>> +	__u32	stx_atomic_write_unit_min;
>> +	__u32	stx_atomic_write_unit_max;
>> +	__u32   stx_atomic_write_segments_max;
> 
> Let's add one liner for each of these fields similar to how it was done
> for others?
> 
> /* Minimum supported atomic write length in bytes */
> /* Maximum supported atomic write length in bytes */
> /* Maximum no. of segments (iovecs?) supported for atomic write */

ok

> 
> 
>> +	__u32   __spare1;
>> +	/* 0xb0 */
>> +	__u64	__spare3[10];	/* Spare space for future expansion */
>>   	/* 0x100 */
>>   };
>>   
>> @@ -155,6 +160,7 @@ struct statx {
>>   #define STATX_MNT_ID		0x00001000U	/* Got stx_mnt_id */
>>   #define STATX_DIOALIGN		0x00002000U	/* Want/got direct I/O alignment info */
>>   #define STATX_MNT_ID_UNIQUE	0x00004000U	/* Want/got extended stx_mount_id */
>> +#define STATX_WRITE_ATOMIC	0x00008000U	/* Want/got atomic_write_* fields */
>>   
>>   #define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */
>>   
>> @@ -190,6 +196,7 @@ struct statx {
>>   #define STATX_ATTR_MOUNT_ROOT		0x00002000 /* Root of a mount */
>>   #define STATX_ATTR_VERITY		0x00100000 /* [I] Verity protected file */
>>   #define STATX_ATTR_DAX			0x00200000 /* File is currently in DAX state */
>> +#define STATX_ATTR_WRITE_ATOMIC		0x00400000 /* File supports atomic write operations */
>>   
>>   
>>   #endif /* _UAPI_LINUX_STAT_H */
>> -- 
>> 2.31.1

Thanks,
John