Re: [RFC v2 14/14] xfs: enable block size larger than page size support

[Dave Chinner <david@fromorbit.com>]

On Tue, Feb 13, 2024 at 10:37:13AM +0100, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.com>
> 
> Page cache now has the ability to have a minimum order when allocating
> a folio which is a prerequisite to add support for block size > page
> size. Enable it in XFS under CONFIG_XFS_LBS.
> 
> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> ---
>  fs/xfs/xfs_icache.c | 8 ++++++--
>  fs/xfs/xfs_super.c  | 8 +++-----
>  2 files changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index dba514a2c84d..9de81caf7ad4 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -73,6 +73,7 @@ xfs_inode_alloc(
>  	xfs_ino_t		ino)
>  {
>  	struct xfs_inode	*ip;
> +	int			min_order = 0;
>  
>  	/*
>  	 * XXX: If this didn't occur in transactions, we could drop GFP_NOFAIL
> @@ -88,7 +89,8 @@ xfs_inode_alloc(
>  	/* VFS doesn't initialise i_mode or i_state! */
>  	VFS_I(ip)->i_mode = 0;
>  	VFS_I(ip)->i_state = 0;
> -	mapping_set_large_folios(VFS_I(ip)->i_mapping);
> +	min_order = max(min_order, ilog2(mp->m_sb.sb_blocksize) - PAGE_SHIFT);
> +	mapping_set_folio_orders(VFS_I(ip)->i_mapping, min_order, MAX_PAGECACHE_ORDER);

That's pretty nasty. You're using max() to hide underflow in the
subtraction to clamp the value to zero. And you don't need ilog2()
because we have the log of the block size in the superblock already.

	int			min_order = 0;
	.....
	if (mp->m_sb.sb_blocksize > PAGE_SIZE)
		min_order = mp->m_sb.sb_blocklog - PAGE_SHIFT;

But, really why recalculate this -constant- on every inode
allocation?  That's a very hot path, so this should be set in the
M_IGEO(mp) structure (mp->m_ino_geo) at mount time and then the code
is simply:

	mapping_set_folio_orders(VFS_I(ip)->i_mapping,
			M_IGEO(mp)->min_folio_order, MAX_PAGECACHE_ORDER);

We already access the M_IGEO(mp) structure every inode allocation,
so there's little in way of additional cost here....

> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index 5a2512d20bd0..6a3f0f6727eb 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -1625,13 +1625,11 @@ xfs_fs_fill_super(
>  		goto out_free_sb;
>  	}
>  
> -	/*
> -	 * Until this is fixed only page-sized or smaller data blocks work.
> -	 */
> -	if (mp->m_sb.sb_blocksize > PAGE_SIZE) {
> +	if (!IS_ENABLED(CONFIG_XFS_LBS) && mp->m_sb.sb_blocksize > PAGE_SIZE) {
>  		xfs_warn(mp,
>  		"File system with blocksize %d bytes. "
> -		"Only pagesize (%ld) or less will currently work.",
> +		"Only pagesize (%ld) or less will currently work. "
> +		"Enable Experimental CONFIG_XFS_LBS for this support",
>  				mp->m_sb.sb_blocksize, PAGE_SIZE);
>  		error = -ENOSYS;
>  		goto out_free_sb;

This should just issue a warning if bs > ps.

	if (mp->m_sb.sb_blocksize > PAGE_SIZE) {
  		xfs_warn(mp,
"EXPERIMENTAL: Filesystem with Large Block Size (%d bytes) enabled.",
			mp->m_sb.sb_blocksize);
	}

-Dave.
-- 
Dave Chinner
david@fromorbit.com