linux-kernel - Re: [PATCH v8 01/10] fs: Allow fine-grained control of folio sizes

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Message-ID: <20240709210829.dgm6dsirkry3fgu6@quentin>
Date: Tue, 9 Jul 2024 21:08:29 +0000
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: david@...morbit.com, willy@...radead.org, ryan.roberts@....com,
	linux-kernel@...r.kernel.org, yang@...amperecomputing.com,
	linux-mm@...ck.org, john.g.garry@...cle.com,
	linux-fsdevel@...r.kernel.org, hare@...e.de, p.raghav@...sung.com,
	mcgrof@...nel.org, gost.dev@...sung.com, cl@...amperecomputing.com,
	linux-xfs@...r.kernel.org, hch@....de, Zi Yan <zi.yan@...t.com>,
	akpm@...ux-foundation.org, chandan.babu@...cle.com
Subject: Re: [PATCH v8 01/10] fs: Allow fine-grained control of folio sizes

> > 
> > - We make THP an explicit dependency for XFS:
> > 
> > diff --git a/fs/xfs/Kconfig b/fs/xfs/Kconfig
> > index d41edd30388b7..be2c1c0e9fe8b 100644
> > --- a/fs/xfs/Kconfig
> > +++ b/fs/xfs/Kconfig
> > @@ -5,6 +5,7 @@ config XFS_FS
> >         select EXPORTFS
> >         select LIBCRC32C
> >         select FS_IOMAP
> > +       select TRANSPARENT_HUGEPAGE
> >         help
> >           XFS is a high performance journaling filesystem which originated
> >           on the SGI IRIX platform.  It is completely multi-threaded, can
> > 
> > OR
> > 
> > We create a helper in page cache that FSs can use to check if a specific
> > order can be supported at mount time:
> 
> I like this solution better; if XFS is going to drop support for o[ld]d
> architectures I think we need /some/ sort of notice period.  Or at least
> a better story than "we want to support 64k fsblocks on x64 so we're
> withdrawing support even for 4k fsblocks and smallish filesystems on
> m68k".
> 
> You probably don't want bs>ps support to block on some arcane discussion
> about 32-bit, right? ;)
> 

:)

> > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> > index 14e1415f7dcf..9be775ef11a5 100644
> > --- a/include/linux/pagemap.h
> > +++ b/include/linux/pagemap.h
> > @@ -374,6 +374,14 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
> >  #define MAX_XAS_ORDER          (XA_CHUNK_SHIFT * 2 - 1)
> >  #define MAX_PAGECACHE_ORDER    min(MAX_XAS_ORDER, PREFERRED_MAX_PAGECACHE_ORDER)
> >  
> > +
> > +static inline unsigned int mapping_max_folio_order_supported()
> > +{
> > +    if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
> > +      return 0;
> 
> Shouldn't this line be indented by two tabs, not six spaces?
> 
> > +    return MAX_PAGECACHE_ORDER;
> > +}
> 
> Alternately, should this return the max folio size in bytes?
> 
> static inline size_t mapping_max_folio_size(void)
> {
> 	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
> 		return 1U << (PAGE_SHIFT + MAX_PAGECACHE_ORDER);
> 	return PAGE_SIZE;
> }

We already have mapping_max_folio_size(mapping) which returns the
maximum folio order set for that mapping. So this could be called as
mapping_max_folio_size_supported().

So we could just have mapping_max_folio_size_supported() instead of
having mapping_max_folio_order_supported as you suggest.

> 
> Then the validation looks like:
> 
> 	const size_t	max_folio_size = mapping_max_folio_size();
> 
> 	if (mp->m_sb.sb_blocksize > max_folio_size) {
> 		xfs_warn(mp,
>  "block size (%u bytes) not supported; maximum folio size is %u.",
> 				mp->m_sb.sb_blocksize, max_folio_size);
> 		error = -ENOSYS;
> 		goto out_free_sb;
> 	}
> 
> (Don't mind me bikeshedding here.)
> 
> > +
> > 
> > 
> > diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> > index b8a93a8f35cac..e2be8743c2c20 100644
> > --- a/fs/xfs/xfs_super.c
> > +++ b/fs/xfs/xfs_super.c
> > @@ -1647,6 +1647,15 @@ xfs_fs_fill_super(
> >                         goto out_free_sb;
> >                 }
> >  
> > +               if (mp->m_sb.sb_blocklog - PAGE_SHIFT >
> > +                   mapping_max_folio_order_supported()) {
> > +                       xfs_warn(mp,
> > +"Block Size (%d bytes) is not supported. Check MAX_PAGECACHE_ORDER",
> > +                       mp->m_sb.sb_blocksize);
> 
> You might as well print MAX_PAGECACHE_ORDER here to make analysis
> easier on less-familiar architectures:

Yes!

> 
> 			xfs_warn(mp,
>  "block size (%d bytes) is not supported; max folio size is %u.",
> 					mp->m_sb.sb_blocksize,
> 					1U << mapping_max_folio_order_supported());
> 
> (I wrote this comment first.)

> 
> --D
> 
> > +                       error = -ENOSYS;
> > +                       goto out_free_sb;
> > +               }
> > +
> >                 xfs_warn(mp,
> >  "EXPERIMENTAL: V5 Filesystem with Large Block Size (%d bytes) enabled.",
> >                         mp->m_sb.sb_blocksize);
> > 
> > 
> > --
> > Pankaj