linux-ext4 - Re: Directories

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Message-ID: <p73bqoccehs.fsf@verdi.suse.de>
Date:	16 Oct 2006 20:17:03 +0200
From:	Andi Kleen <ak@...e.de>
To:	Steve Lord <lord@....org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org, xfs@....sgi.com
Subject: Re: Directories > 2GB

Steve Lord <lord@....org> writes:

> 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.

# grep -i vmalloc /proc/meminfo 
VmallocTotal: 34359738367 kB

At least on 64bit systems it would be reasonable to keep a large
number of directories mapped this way over a longer time. vmap() space is 
cheap there.

-Andi
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