Re: [PATCH v3 2/6] xfs: add helpers to pack AG prediction info for per-folio tracking

[Kundan Kumar <kundan.kumar@samsung.com>]

On 1/29/2026 6:15 AM, Darrick J. Wong wrote:
> On Fri, Jan 16, 2026 at 03:38:14PM +0530, Kundan Kumar wrote:
>> Introduce helper routines to pack and unpack AG prediction metadata
>> for folios. This provides a compact and self-contained representation
>> for AG tracking.
>>
>> The packed layout uses:
>>   - bit 31	: valid
>>   - bit 24-30	: iomap type
>>   - bit 0-23	: AG number
> 
> There are only 5 iomap types, why do you need 7 bits for that?
> 
> Also, can you store more bits on a 64-bit system to avoid truncating the
> AG number?
> 
> --D

I’ll reduce the type field to 3 bits (8 values).

For the AG number, I can drop the artificial 24-bit cap by packing into 
an unsigned long and storing it via xa_mk_value(), which provides ~60 
bits on 64-bit systems and ~28 bits on 32-bit systems.

> 
>> Signed-off-by: Kundan Kumar <kundan.kumar@samsung.com>
>> Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
>> ---
>>   fs/xfs/xfs_iomap.h | 31 +++++++++++++++++++++++++++++++
>>   1 file changed, 31 insertions(+)
>>
>> diff --git a/fs/xfs/xfs_iomap.h b/fs/xfs/xfs_iomap.h
>> index ebcce7d49446..eaf4513f6759 100644
>> --- a/fs/xfs/xfs_iomap.h
>> +++ b/fs/xfs/xfs_iomap.h
>> @@ -12,6 +12,37 @@ struct xfs_inode;
>>   struct xfs_bmbt_irec;
>>   struct xfs_zone_alloc_ctx;
>>   
>> +/* pack prediction in a u32 stored in xarray */
>> +#define XFS_AGP_VALID_SHIFT 31
>> +#define XFS_AGP_TYPE_SHIFT 24
>> +#define XFS_AGP_TYPE_MASK 0x7fu
>> +#define XFS_AGP_AGNO_MASK 0x00ffffffu
>> +
>> +static inline u32 xfs_agp_pack(u32 agno, u8 iomap_type, bool valid)
>> +{
>> +	u32 v = agno & XFS_AGP_AGNO_MASK;
>> +
>> +	v |= ((u32)iomap_type & XFS_AGP_TYPE_MASK) << XFS_AGP_TYPE_SHIFT;
>> +	if (valid)
>> +		v |= (1u << XFS_AGP_VALID_SHIFT);
>> +	return v;
>> +}
>> +
>> +static inline bool xfs_agp_valid(u32 v)
>> +{
>> +	return v >> XFS_AGP_VALID_SHIFT;
>> +}
>> +
>> +static inline u32 xfs_agp_agno(u32 v)
>> +{
>> +	return v & XFS_AGP_AGNO_MASK;
>> +}
>> +
>> +static inline u8 xfs_agp_type(u32 v)
>> +{
>> +	return (u8)((v >> XFS_AGP_TYPE_SHIFT) & XFS_AGP_TYPE_MASK);
>> +}
>> +
>>   int xfs_iomap_write_direct(struct xfs_inode *ip, xfs_fileoff_t offset_fsb,
>>   		xfs_fileoff_t count_fsb, unsigned int flags,
>>   		struct xfs_bmbt_irec *imap, u64 *sequence);
>> -- 
>> 2.25.1
>>
>>
>