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

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Date: Wed, 14 Feb 2024 17:35:49 +0100
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Dave Chinner <david@...morbit.com>
Cc: linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	mcgrof@...nel.org, gost.dev@...sung.com, akpm@...ux-foundation.org, 
	kbusch@...nel.org, djwong@...nel.org, chandan.babu@...cle.com, p.raghav@...sung.com, 
	linux-kernel@...r.kernel.org, hare@...e.de, willy@...radead.org, linux-mm@...ck.org
Subject: Re: [RFC v2 14/14] xfs: enable block size larger than page size
 support

> >  	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;
how is it underflowing if I am comparing two values of type int?

> 
> 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);
> 

That is a good idea. I will add this change in the next revision.

> 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);
> 	}

Yes! Luis already told me to add a warning here but I missed it before
sending the patches out.

> 
> -Dave.
> -- 
> Dave Chinner
> david@...morbit.com