Re: [PATCH RFC 08/10] xfs: Commit CoW-based atomic writes atomically

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

On 05/02/2025 19:47, Darrick J. Wong wrote:
> On Tue, Feb 04, 2025 at 12:01:25PM +0000, John Garry wrote:
>> When completing a CoW-based write, each extent range mapping update is
>> covered by a separate transaction.
>>
>> For a CoW-based atomic write, all mappings must be changed at once, so
>> change to use a single transaction.
>>
>> Signed-off-by: John Garry <john.g.garry@oracle.com>
>> ---
>>   fs/xfs/xfs_file.c    |  5 ++++-
>>   fs/xfs/xfs_reflink.c | 48 ++++++++++++++++++++++++++++++++++++++++++++
>>   fs/xfs/xfs_reflink.h |  3 +++
>>   3 files changed, 55 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
>> index 12af5cdc3094..170d7891f90d 100644
>> --- a/fs/xfs/xfs_file.c
>> +++ b/fs/xfs/xfs_file.c
>> @@ -527,7 +527,10 @@ xfs_dio_write_end_io(
>>   	nofs_flag = memalloc_nofs_save();
>>   
>>   	if (flags & IOMAP_DIO_COW) {
>> -		error = xfs_reflink_end_cow(ip, offset, size);
>> +		if (iocb->ki_flags & IOCB_ATOMIC)
>> +			error = xfs_reflink_end_atomic_cow(ip, offset, size);
>> +		else
>> +			error = xfs_reflink_end_cow(ip, offset, size);
>>   		if (error)
>>   			goto out;
>>   	}
>> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
>> index dbce333b60eb..60c986300faa 100644
>> --- a/fs/xfs/xfs_reflink.c
>> +++ b/fs/xfs/xfs_reflink.c
>> @@ -990,6 +990,54 @@ xfs_reflink_end_cow(
>>   		trace_xfs_reflink_end_cow_error(ip, error, _RET_IP_);
>>   	return error;
>>   }
>> +int
>> +xfs_reflink_end_atomic_cow(
>> +	struct xfs_inode		*ip,
>> +	xfs_off_t			offset,
>> +	xfs_off_t			count)
>> +{
>> +	xfs_fileoff_t			offset_fsb;
>> +	xfs_fileoff_t			end_fsb;
>> +	int				error = 0;
>> +	struct xfs_mount		*mp = ip->i_mount;
>> +	struct xfs_trans		*tp;
>> +	unsigned int			resblks;
>> +	bool				commit = false;
>> +
>> +	trace_xfs_reflink_end_cow(ip, offset, count);
>> +
>> +	offset_fsb = XFS_B_TO_FSBT(ip->i_mount, offset);
>> +	end_fsb = XFS_B_TO_FSB(ip->i_mount, offset + count);
>> +
>> +	resblks = XFS_NEXTENTADD_SPACE_RES(ip->i_mount,
>> +				(unsigned int)(end_fsb - offset_fsb),
>> +				XFS_DATA_FORK);
>> +
>> +	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, resblks, 0,
> 
> xfs gained reflink support for realtime volumes in 6.14-rc1, so you now
> have to calculate for that in here too.
> 
>> +			XFS_TRANS_RESERVE, &tp);
>> +	if (error)
>> +		return error;
>> +
>> +	xfs_ilock(ip, XFS_ILOCK_EXCL);
>> +	xfs_trans_ijoin(tp, ip, 0);
>> +
>> +	while (end_fsb > offset_fsb && !error)
>> +		error = xfs_reflink_end_cow_extent_locked(ip, &offset_fsb,
>> +						end_fsb, tp, &commit);
> 
> Hmm.  Attaching intent items to a transaction consumes space in that
> transaction, so we probably ought to limit the amount that we try to do
> here.  Do you know what that limit is?  I don't, 

nor do I ...

> but it's roughly
> tr_logres divided by the average size of a log intent item.

So you have a ballpark figure on the average size of a log intent item, 
or an idea on how to get it?

> 
> This means we need to restrict the size of an untorn write to a
> double-digit number of fsblocks for safety.

Sure, but won't we also still be liable to suffer the same issue which 
was fixed in commit d6f215f359637?

> 
> The logic in here looks reasonable though.
> 

Thanks,
John

> --D
> 
>> +
>> +	if (error || !commit)
>> +		goto out_cancel;
>> +
>> +	if (error)
>> +		trace_xfs_reflink_end_cow_error(ip, error, _RET_IP_);
>> +	error = xfs_trans_commit(tp);
>> +	xfs_iunlock(ip, XFS_ILOCK_EXCL);
>> +	return error;
>> +out_cancel:
>> +	xfs_trans_cancel(tp);
>> +	xfs_iunlock(ip, XFS_ILOCK_EXCL);
>> +	return error;
>> +}
>>   
>>   /*
>>    * Free all CoW staging blocks that are still referenced by the ondisk refcount
>> diff --git a/fs/xfs/xfs_reflink.h b/fs/xfs/xfs_reflink.h
>> index ef5c8b2398d8..2c3b096c1386 100644
>> --- a/fs/xfs/xfs_reflink.h
>> +++ b/fs/xfs/xfs_reflink.h
>> @@ -45,6 +45,9 @@ extern int xfs_reflink_cancel_cow_range(struct xfs_inode *ip, xfs_off_t offset,
>>   		xfs_off_t count, bool cancel_real);
>>   extern int xfs_reflink_end_cow(struct xfs_inode *ip, xfs_off_t offset,
>>   		xfs_off_t count);
>> +		int
>> +xfs_reflink_end_atomic_cow(struct xfs_inode *ip, xfs_off_t offset,
>> +		xfs_off_t count);
>>   extern int xfs_reflink_recover_cow(struct xfs_mount *mp);
>>   extern loff_t xfs_reflink_remap_range(struct file *file_in, loff_t pos_in,
>>   		struct file *file_out, loff_t pos_out, loff_t len,
>> -- 
>> 2.31.1
>>
>>