linux-ext4 - Re: ext4dev build failure on mips: "empty_zero

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Message-ID: <20080605183854.GN25477@mit.edu>
Date:	Thu, 5 Jun 2008 14:38:54 -0400
From:	Theodore Tso <tytso@....EDU>
To:	Dmitri Vorobiev <dmitri.vorobiev@...ial.fi>
Cc:	Martin Michlmayr <tbm@...ius.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Dmitri Vorobiev <dmitri.vorobiev@...il.com>,
	linux-mips@...ux-mips.org, linux-ext4@...r.kernel.org
Subject: Re: ext4dev build failure on mips: "empty_zero_page" undefined

On Thu, Jun 05, 2008 at 02:22:57PM +0300, Dmitri Vorobiev wrote:
> 
> It looks like the discussion related to this issue has faded out. Ralf
> seemed to have some objections to using ZERO_PAGE() outside of the
> context of getting a user-mapped page, but I think that ext4 driver is
> still doing that.
> 

I thought I had sent a reply indicating that yes, we are using
ZERO_PAGE outside of getting a user-mapped page, because this is
happening when we need to write zero's to directly to a filesystem
block.  This case arises when we have a file which contains
preallocated space that has not yet been initialized, and the user
program seeks into the middle of the unitialized extent range, and
writes into the middle of that space.

In some cases, it is more efficient to zero out a small range of
blocks on disk rather than splitting the extent in the middle.  We
could explicitly allocate a page, and zero it out, and use it to write
zeros from the ext4 filesystem, code, but that seems silly, given that
ZERO_PAGE exists and is available on all other architectures.

Cristoph Hellwig had complained about the use of ZERO_PAGE, but when I
gave him the above explanation, he agreed that this was indeed
probably the best way to do things.

If you really insist I suppose we could have a MIPS specific patch
where we allocate a 4k page and zero it, so we can use it from our
kernel code because you don't want to export and make available the
ZERO_PAGE that gets used by the rest of the kernel, but that seems
awfully silly, and would be a waste of 4k of memory.....  Someone from
MIPS land would have to test it, as well, as I dont think any of the
ext4 developers have access to a MIPS platform.

						- Ted
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