linux-kernel - Re: [PATCH 3/17] afs: convert afs_dir_get_page to read_kmap

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Date:	Thu, 12 Apr 2007 20:43:11 +0100
From:	David Howells <dhowells@...hat.com>
To:	"Nate Diller" <nate.diller@...il.com>
Cc:	"Andrew Morton" <akpm@...l.org>,
	"Alexander Viro" <viro@...iv.linux.org.uk>,
	"Christoph Hellwig" <hch@...radead.org>,
	"Roman Zippel" <zippel@...ux-m68k.org>,
	"Mikulas Patocka" <mikulas@...ax.karlin.mff.cuni.cz>,
	"David Woodhouse" <dwmw2@...radead.org>,
	"Dave Kleikamp" <shaggy@...tin.ibm.com>,
	"Anton Altaparmakov" <aia21@...tab.net>,
	"Evgeniy Dushistov" <dushistov@...l.ru>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	reiserfs-dev@...esys.com
Subject: Re: [PATCH 3/17] afs: convert afs_dir_get_page to read_kmap_page 

Nate Diller <nate.diller@...il.com> wrote:

> but that's a lot of code to avoid a single stack allocation.  The
> whole fake file pointer thing still strikes me as a little ugly, and
> you're definitely not the first one who needed this sort of hackery.
> ugh

A better way might be to stick a void * in struct file and pass that through
to readpage() and readpages() instead of the struct file *.  That way, anyone
who wants the traditional arrangement can just point that extra void * at the
struct file.

Of course, I'm in favour of making it a struct key * like this:

	struct address_space_operations {
	...
		int (*readpage)(struct key *, struct page *);
	...
		int (*readpages)(struct key *, struct address_space *,
				struct list_head *, unsigned);
	...
	};

	struct file {
	...
		struct key *f_key;
	};

	struct page *filemap_nopage(struct vm_area_struct *area, ...)
	{
	...
		struct file *file = area->vm_file;
	...
		error = mapping->a_ops->readpage(file->f_key, page);
	...
	}
	
But I'm not sure the NFS crew, for instance, would be happy with that.  Maybe
passing file->private_data through would do.  That's basically what NFS and
FUSE, for instance, want, and it would also do for AFS.

David
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