linux-kernel - Re: [PATCH 1/4] fs/anon_inodes: new interface to create new inode

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Date:	Wed, 29 Jul 2015 11:50:03 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Gioh Kim <gioh.kim@....com>
Cc:	jlayton@...chiereds.net, bfields@...ldses.org, vbabka@...e.cz,
	iamjoonsoo.kim@....com, viro@...iv.linux.org.uk, mst@...hat.com,
	koct9i@...il.com, minchan@...nel.org, aquini@...hat.com,
	linux-fsdevel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	linux-mm@...ck.org, dri-devel@...ts.freedesktop.org,
	akpm@...ux-foundation.org, Gioh Kim <gurugio@...mail.net>
Subject: Re: [PATCH 1/4] fs/anon_inodes: new interface to create new inode

On Mon, Jul 13, 2015 at 05:35:16PM +0900, Gioh Kim wrote:
> From: Gioh Kim <gurugio@...mail.net>
> 
> The anon_inodes has already complete interfaces to create manage
> many anonymous inodes but don't have interface to get
> new inode. Other sub-modules can create anonymous inode
> without creating and mounting it's own pseudo filesystem.
> 
> Signed-off-by: Gioh Kim <gioh.kim@....com>
> Acked-by: Rafael Aquini <aquini@...hat.com>

This is my first run through the series so I'm going to miss details but
this patch confuses me a little. You create an inode to associate with
the balloon dev_info so that page->mapping can be assigned. It's only the
mapping you care about for the aops so why are multiple inodes required? A
driver should be able to share and reference count a single inode. The
motivation to do it that way would be to reduce memory consumption and
this series is motivated by embedded platforms.

anon_inode_getfd has the following

 * Creates a new file by hooking it on a single inode. This is useful for files
 * that do not need to have a full-fledged inode in order to operate correctly.
 * All the files created with anon_inode_getfd() will share a single inode,
 * hence saving memory and avoiding code duplication for the file/inode/dentry
 * setup.  Returns new descriptor or an error code.

If all we care about the inode is the aops then it would follow that
anon_inode_getfd() is ideal. The tradeoff is reference counting overhead.
The changelog needs to explain why anon_inode_getfd() cannot be used.

-- 
Mel Gorman
SUSE Labs
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