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Date:   Fri, 22 May 2020 07:55:23 +1000
From:   Dave Chinner <david@fromorbit.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 07/36] fs: Add a filesystem flag for large pages
Message-ID: <20200521215523.GR2005@dread.disaster.area>
References: <20200515131656.12890-1-willy@infradead.org>
 <20200515131656.12890-8-willy@infradead.org>
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On Fri, May 15, 2020 at 06:16:27AM -0700, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> The page cache needs to know whether the filesystem supports pages >
> PAGE_SIZE.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  include/linux/fs.h      | 1 +
>  include/linux/pagemap.h | 5 +++++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 55c743925c40..777783c8760b 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2241,6 +2241,7 @@ struct file_system_type {
>  #define FS_HAS_SUBTYPE		4
>  #define FS_USERNS_MOUNT		8	/* Can be mounted by userns root */
>  #define FS_DISALLOW_NOTIFY_PERM	16	/* Disable fanotify permission events */
> +#define FS_LARGE_PAGES		8192	/* Remove once all fs converted */
>  #define FS_RENAME_DOES_D_MOVE	32768	/* FS will handle d_move() during rename() internally. */
>  	int (*init_fs_context)(struct fs_context *);
>  	const struct fs_parameter_spec *parameters;
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 36bfc9d855bb..c6db74b5e62f 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -116,6 +116,11 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
>  	m->gfp_mask = mask;
>  }
>  
> +static inline bool mapping_large_pages(struct address_space *mapping)
> +{
> +	return mapping->host->i_sb->s_type->fs_flags & FS_LARGE_PAGES;
> +}

If you've got to dereference 4 layers deep to check a behaviour
flag, the object needs it's own flag.  Can you just propagate this
to the address space when the inode is instantiated and the address
space initialised?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com