From: Steve Lord <lord@xfs.org>
Subject: Re: Directories > 2GB
Date: Wed, 11 Oct 2006 11:49:10 -0500
Message-ID: <452D2086.2020204@xfs.org>
References: <20061004165655.GD22010@schatzie.adilger.int> <452AC4BE.6090905@xfs.org> <20061010015512.GQ11034@melbourne.sgi.com> <452B0240.60203@xfs.org> <20061010091904.GA395@infradead.org> <20061010233124.GX11034@melbourne.sgi.com>
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Cc: Christoph Hellwig <hch@infradead.org>,
	linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-kernel@vger.kernel.org, xfs@oss.sgi.com
Return-path: <linux-fsdevel-owner@vger.kernel.org>
To: David Chinner <dgc@sgi.com>
In-Reply-To: <20061010233124.GX11034@melbourne.sgi.com>
Sender: linux-fsdevel-owner@vger.kernel.org
List-Id: linux-ext4.vger.kernel.org

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