Date: Wed, 1 May 2024 11:03:06 +0100
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, willy@...radead.org,
        axboe@...nel.dk, martin.petersen@...cle.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        tytso@....edu, jbongio@...gle.com, ojaswin@...ux.ibm.com,
        ritesh.list@...il.com, mcgrof@...nel.org, p.raghav@...sung.com,
        linux-xfs@...r.kernel.org, catherine.hoang@...cle.com
Subject: Re: [PATCH v3 08/21] xfs: Introduce FORCEALIGN inode flag


>> +/* Validate the forcealign inode flag */
>> +xfs_failaddr_t
>> +xfs_inode_validate_forcealign(
>> +	struct xfs_mount	*mp,
>> +	uint16_t		mode,
> 
> 	umode_t			mode,

ok. BTW, other functions like xfs_inode_validate_extsize() use uint16_t

> 
>> +	uint16_t		flags,
>> +	uint32_t		extsize,
>> +	uint32_t		cowextsize)
> 
> extent sizes are xfs_extlen_t types.

ok

> 
>> +{
>> +	/* superblock rocompat feature flag */
>> +	if (!xfs_has_forcealign(mp))
>> +		return __this_address;
>> +
>> +	/* Only regular files and directories */
>> +	if (!S_ISDIR(mode) && !S_ISREG(mode))
>> +		return __this_address;
>> +
>> +	/* Doesn't apply to realtime files */
>> +	if (flags & XFS_DIFLAG_REALTIME)
>> +		return __this_address;
> 
> Why not? A rt device with an extsize of 1 fsb could make use of
> forced alignment just like the data device to allow larger atomic
> writes to be done. I mean, just because we haven't written the code
> to do this yet doesn't mean it is an illegal on-disk format state.

ok, so where is a better place to disallow forcealign for RT now (since 
we have not written the code to support it nor verified it)?

> 
>> +	/* Requires a non-zero power-of-2 extent size hint */
>> +	if (extsize == 0 || !is_power_of_2(extsize) ||
>> +	    (mp->m_sb.sb_agblocks % extsize))
>> +		return __this_address;
> 
> Please do these as indiviual checks with their own fail address.

ok

> That way we can tell which check failed from the console output.
> Also, the agblocks check is already split out below, so it's being
> checked twice...
> 
> Also, why does force-align require a power-of-2 extent size? Why
> does it require the extent size to be an exact divisor of the AG
> size? Aren't these atomic write alignment restrictions? i.e.
> shouldn't these only be enforced when the atomic writes inode flag
> is set?

With regards the power-of-2 restriction, I think that the code changes 
are going to become a lot more complex if we don't enforce this for 
forcealign.

For example, consider xfs_file_dio_write(), where we check for an 
unaligned write based on forcealign extent mask. It's much simpler to 
rely on a power-of-2 size. And same for iomap extent zeroing.

So then it can be asked, for what reason do we want to support 
unorthodox, non-power-of-2 sizes? Who would want this?

As for AG size, again I think that it is required to be aligned to the 
forcealign extsize. As I remember, when converting from an FSB to a DB, 
if the AG itself is not aligned to the forcealign extsize, then the DB 
will not be aligned to the forcealign extsize. More below...

> 
>> +	/* Requires agsize be a multiple of extsize */
>> +	if (mp->m_sb.sb_agblocks % extsize)
>> +		return __this_address;
>> +
>> +	/* Requires stripe unit+width (if set) be a multiple of extsize */
>> +	if ((mp->m_dalign && (mp->m_dalign % extsize)) ||
>> +	    (mp->m_swidth && (mp->m_swidth % extsize)))
>> +		return __this_address;
> 
> Again, this is an atomic write constraint, isn't it?

So why do we want forcealign? It is to only align extent FSBs? Or to 
align extents to DBs? I would have thought the latter. If so, it seems 
sensible to do this check also.

> 
>> +	/* Requires no cow extent size hint */
>> +	if (cowextsize != 0)
>> +		return __this_address;
> 
> What if it's a reflinked file?

Yeah, I think that we want to disallow that.

> 
> .....
> 
>> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
>> index d0e2cec6210d..d1126509ceb9 100644
>> --- a/fs/xfs/xfs_ioctl.c
>> +++ b/fs/xfs/xfs_ioctl.c
>> @@ -1110,6 +1110,8 @@ xfs_flags2diflags2(
>>   		di_flags2 |= XFS_DIFLAG2_DAX;
>>   	if (xflags & FS_XFLAG_COWEXTSIZE)
>>   		di_flags2 |= XFS_DIFLAG2_COWEXTSIZE;
>> +	if (xflags & FS_XFLAG_FORCEALIGN)
>> +		di_flags2 |= XFS_DIFLAG2_FORCEALIGN;
>>   
>>   	return di_flags2;
>>   }
>> @@ -1146,6 +1148,22 @@ xfs_ioctl_setattr_xflags(
>>   	if (i_flags2 && !xfs_has_v3inodes(mp))
>>   		return -EINVAL;
>>   
>> +	/*
>> +	 * Force-align requires a nonzero extent size hint and a zero cow
>> +	 * extent size hint.  It doesn't apply to realtime files.
>> +	 */
>> +	if (fa->fsx_xflags & FS_XFLAG_FORCEALIGN) {
>> +		if (!xfs_has_forcealign(mp))
>> +			return -EINVAL;
>> +		if (fa->fsx_xflags & FS_XFLAG_COWEXTSIZE)
>> +			return -EINVAL;
>> +		if (!(fa->fsx_xflags & (FS_XFLAG_EXTSIZE |
>> +					FS_XFLAG_EXTSZINHERIT)))
>> +			return -EINVAL;
>> +		if (fa->fsx_xflags & FS_XFLAG_REALTIME)
>> +			return -EINVAL;
>> +	}
> 
> What about if the file already has shared extents on it (i.e.
> reflinked or deduped?)

At the top of the function we have this check for RT:

	if (rtflag != XFS_IS_REALTIME_INODE(ip)) {
		/* Can't change realtime flag if any extents are allocated. */
		if (ip->i_df.if_nextents || ip->i_delayed_blks)
			return -EINVAL;
	}

Would expanding that check for forcealign also suffice? Indeed, later in 
this series I expanded this check to cover atomicwrites (when I really 
intended it for forcealign).

> 
> Also, why is this getting checked here instead of in
> xfs_ioctl_setattr_check_extsize()?
> 
> 
>> @@ -1263,7 +1283,19 @@ xfs_ioctl_setattr_check_extsize(
>>   	failaddr = xfs_inode_validate_extsize(ip->i_mount,
>>   			XFS_B_TO_FSB(mp, fa->fsx_extsize),
>>   			VFS_I(ip)->i_mode, new_diflags);
>> -	return failaddr != NULL ? -EINVAL : 0;
>> +	if (failaddr)
>> +		return -EINVAL;
>> +
>> +	if (new_diflags2 & XFS_DIFLAG2_FORCEALIGN) {
>> +		failaddr = xfs_inode_validate_forcealign(ip->i_mount,
>> +				VFS_I(ip)->i_mode, new_diflags,
>> +				XFS_B_TO_FSB(mp, fa->fsx_extsize),
>> +				XFS_B_TO_FSB(mp, fa->fsx_cowextsize));
>> +		if (failaddr)
>> +			return -EINVAL;
>> +	}
> 
> Oh, it's because you're trying to use on-disk format validation
> routines for user API validation. That, IMO, is a bad idea because
> the on-disk format and kernel/user APIs should not be tied
> together as they have different constraints and error conditions.
> 
> That also explains why xfs_inode_validate_forcealign() doesn't just
> get passed the inode to validate - it's because you want to pass
> information from the user API to it. This results in sub-optimal
> code for both on-disk format validation and user API validation.
> 
> Can you please separate these and put all the force align user API
> validation checks in the one function?
> 

ok, fine. But it would be good to have clarification on function of 
forcealign, above, i.e. does it always align extents to disk blocks?

Thanks,
John

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