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Re: Directories > 2GB

To: David Chinner <dgc@xxxxxxx>
Subject: Re: Directories > 2GB
From: Steve Lord <lord@xxxxxxx>
Date: Wed, 11 Oct 2006 11:49:10 -0500
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
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David Chinner wrote:
On Tue, Oct 10, 2006 at 10:19:04AM +0100, Christoph Hellwig wrote:
On Mon, Oct 09, 2006 at 09:15:28PM -0500, Steve Lord wrote:
Hi Dave,

My recollection is that it used to default to on, it was disabled
because it needs to map the buffer into a single contiguous chunk
of kernel memory. This was placing a lot of pressure on the memory
remapping code, so we made it not default to on as reworking the
code to deal with non contig memory was looking like a major
effort.
Exactly.  The code works but tends to go OOM pretty fast at least
when the dir blocksize code is bigger than the page size.  I should
give the code a spin on my ppc box with 64k pages if it works better
there.

The pagebuf code doesn't use high-order allocations anymore; it uses
scatter lists and remapping to allow physically discontiguous pages
in a multi-page buffer. That is, the pages are sourced via
find_or_create_page() from the address space of the backing device,
and then mapped via vmap() to provide a virtually contigous mapping
of the multi-page buffer.

So I don't think this problem exists anymore...

I was not referring to high order allocations here, but the overhead
of doing address space remapping every time a directory is accessed.

Steve


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