linux-kernel - Re: [PATCH v2 06/14] fs: xfs: Do not free EOF blocks for forcealign

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Date: Thu, 7 Mar 2024 11:38:54 +0000
From: John Garry <john.g.garry@...cle.com>
To: Dave Chinner <david@...morbit.com>
Cc: djwong@...nel.org, hch@....de, viro@...iv.linux.org.uk, brauner@...nel.org,
        jack@...e.cz, chandan.babu@...cle.com, axboe@...nel.dk,
        martin.petersen@...cle.com, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        tytso@....edu, jbongio@...gle.com, ojaswin@...ux.ibm.com,
        ritesh.list@...il.com, linux-block@...r.kernel.org
Subject: Re: [PATCH v2 06/14] fs: xfs: Do not free EOF blocks for forcealign


>>   	 */
>>   	end_fsb = XFS_B_TO_FSB(mp, (xfs_ufsize_t)XFS_ISIZE(ip));
>> -	if (XFS_IS_REALTIME_INODE(ip) && mp->m_sb.sb_rextsize > 1)
>> +
>> +	/* Do not free blocks when forcing extent sizes */
> 
> That comment seems wrong - this just rounds up where we start
> trimming from to be aligned...

ok

> 
>> +	if (xfs_get_extsz(ip) > 1)
>> +		end_fsb = roundup_64(end_fsb, xfs_get_extsz(ip));
>> +	else if (XFS_IS_REALTIME_INODE(ip) && mp->m_sb.sb_rextsize > 1)
>>   		end_fsb = xfs_rtb_roundup_rtx(mp, end_fsb);
> 
> I think this would be better written as:
> 
> 	/*
> 	 * Forced extent alignment requires us to round up where we
> 	 * start trimming from so that result is correctly aligned.
> 	 */
> 	if (xfs_inode_forcealign(ip)) {
> 		if (ip->i_extsize > 1)
> 			end_fsb = roundup_64(end_fsb, ip->i_extsize);
> 		else if (XFS_IS_REALTIME_INODE(ip) && mp->m_sb.sb_rextsize > 1)
> 			end_fsb = xfs_rtb_roundup_rtx(mp, end_fsb);
> 	}
> 
> Because we only want end-fsb alignment when forced alignment is
> required.

But why change current rtvol behaviour?

> 
> Which then makes me wonder: truncate needs this, too, doesn't it?
> And the various fallocate() ops like hole punching and extent
> shifting?
> 

Yes, I would think so. I quickly checked rtvol for truncate and it does 
the round up. I would need to check the relevant code for truncate and 
fallocate for forcealign now.

I do also wonder if we could factor out this rounding up code for 
truncate, facallocate, and whatever else.

>> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
>> index 2c439df8c47f..bbb8886f1d32 100644
>> --- a/fs/xfs/xfs_inode.c
>> +++ b/fs/xfs/xfs_inode.c
>> @@ -65,6 +65,20 @@ xfs_get_extsz_hint(
>>   	return 0;
>>   }
>>   
>> +/*
>> + * Helper function to extract extent size. It will return a power-of-2,
>> + * as forcealign requires this.
>> + */
>> +xfs_extlen_t
>> +xfs_get_extsz(
>> +	struct xfs_inode	*ip)
>> +{
>> +	if (xfs_inode_forcealign(ip) && ip->i_extsize)
>> +		return ip->i_extsize;
>> +
>> +	return 1;
>> +}
> 
> This can go away - if it is needed elsewhere, then I think it would
> be better open coded because it better documents what the code is
> doing...
> 

I would rather get rid of xfs_get_extsz() for sure.

Thanks,
John