linux-kernel - Re: [PATCH] Export shmem_file_setup and shmem

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Message-Id: <1217870748.24714.79.camel@koto.keithp.com>
Date:	Mon, 04 Aug 2008 10:25:48 -0700
From:	Keith Packard <keithp@...thp.com>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	keithp@...thp.com, Nick Piggin <nickpiggin@...oo.com.au>,
	Christoph Hellwig <hch@...radead.org>,
	Eric Anholt <eric@...olt.net>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Export shmem_file_setup and shmem_getpage for DRM-GEM

On Mon, 2008-08-04 at 18:09 +0100, Hugh Dickins wrote:

> Whether such usage conforms to VFS API I'm not so sure: as I understand
> it, it's really for internal use by a filesystem

Sure, but presumably it could even be used by a layered file system?

>  - if it's going to be
> used beyond that, we ought to add a check that the filesystem it's used
> upon really has a ->readpage method (and I'd rather we add such a check
> than you do it at your end, in case we change the implementation later
> to use something other than a ->readpage method - Nick, you'll be
> nauseated to hear I was looking to see if ->fault with a pseudo-vma
> could do it).  But if the layering police are happy with this, I am.

It seems like I should put a check into my code that is kernel version
dependent so that I can't oops if someone tries to use a filesystem that
doesn't have ->readpage.

> But that route is in
> use and well-tested, and only an inefficiency when swapping, so should
> not cause you any problems.

Yeah, swapping performance isn't my primary concern; I looked through
the read_mapping_page codepath and it looked exactly like my existing
code in the fast path, which is why I was able to just delete all of
that from my driver and just call read_mapping_page.

So, when I release the pages from the page cache, I'm currently calling
mark_page_accessed for all pages, and set_page_dirty for pages which may
have been written by the GPU. Are those calls still needed?

-- 
keith.packard@...el.com

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