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

John Garry
2024-02-26T09:07:57Z

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