linux-kernel - Re: [PATCH v2 09/11] xfs: Commit CoW-based atomic writes atomically

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Message-ID: <b2ba8b64-be86-474d-874c-273bbeb4df00@oracle.com>
Date: Tue, 25 Feb 2025 11:11:45 +0000
From: John Garry <john.g.garry@...cle.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: brauner@...nel.org, cem@...nel.org, dchinner@...hat.com, hch@....de,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, ojaswin@...ux.ibm.com,
        ritesh.list@...il.com, martin.petersen@...cle.com, tytso@....edu,
        linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2 09/11] xfs: Commit CoW-based atomic writes atomically

On 24/02/2025 20:20, Darrick J. Wong wrote:
> On Thu, Feb 13, 2025 at 01:56:17PM +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@...cle.com>
>> ---
>>   fs/xfs/xfs_file.c    |  5 ++++-
>>   fs/xfs/xfs_reflink.c | 45 ++++++++++++++++++++++++++++++++++++++++++++
>>   fs/xfs/xfs_reflink.h |  3 +++
>>   3 files changed, 52 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
>> index 9762fa503a41..243640fe4874 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 3dab3ba900a3..d097d33dc000 100644
>> --- a/fs/xfs/xfs_reflink.c
>> +++ b/fs/xfs/xfs_reflink.c
>> @@ -986,6 +986,51 @@ 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;
>> +
>> +	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);
> 
> Use @mp here instead of walking the pointer.

Yes

> 
>> +
>> +	resblks = (end_fsb - offset_fsb) *
>> +			XFS_NEXTENTADD_SPACE_RES(mp, 1, XFS_DATA_FORK);
> 
> How did you arrive at this computation? 

hmmm... you suggested this, but maybe I picked it up incorrectly :)

> The "b" parameter to
> XFS_NEXTENTADD_SPACE_RES is usually the worst case number of mappings
> that you're going to change on this file.  I think that quantity is
> (end_fsb - offset_fsb)?

Can you please check this versus what you suggested in 
https://lore.kernel.org/linux-xfs/20250206215014.GX21808@frogsfrogsfrogs/#t

> 
>> +
>> +	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, resblks, 0,
>> +			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(tp, ip, &offset_fsb,
>> +							end_fsb);
> 
> Overly long line, and the continuation line only needs to be indented
> two more tabs.

ok

> 
>> +
>> +	if (error) {
>> +		trace_xfs_reflink_end_cow_error(ip, error, _RET_IP_);
>> +		goto out_cancel;
>> +	}
>> +	error = xfs_trans_commit(tp);
>> +	xfs_iunlock(ip, XFS_ILOCK_EXCL);
>> +	return 0;
> 
> Why is it ok to drop @error here?  Shouldn't a transaction commit error
> should be reported to the writer thread?
>

I can fix that, as I should not ignore errors from xfs_trans_commit()

Thanks,
John