linux-kernel - Re: [patch] rfc: introduce /dev/hugetlb

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Message-Id: <20070323233917.e5c1a4fc.akpm@linux-foundation.org>
Date:	Fri, 23 Mar 2007 23:39:17 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Ken Chen" <kenchen@...gle.com>
Cc:	"Nish Aravamudan" <nish.aravamudan@...il.com>,
	"Adam Litke" <agl@...ibm.com>,
	"Arjan van de Ven" <arjan@...radead.org>,
	"William Lee Irwin III" <wli@...omorphy.com>,
	"Christoph Hellwig" <hch@...radead.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] rfc: introduce /dev/hugetlb

On Sat, 24 Mar 2007 00:11:32 -0700 "Ken Chen" <kenchen@...gle.com> wrote:

> On 3/23/07, Andrew Morton <akpm@...ux-foundation.org> wrote:
> > a) Ken observes that obtaining private hugetlb memory via hugetlbfs
> >    involves "fuss".
> >
> > b) the libhugetlbfs maintainers then go off and implement a no-fuss way of
> >    doing this.
> 
> Hmm, what started this thread was libhugetlbfs maintainer complained
> how "fuss" it was to create private hugetlb mapping and suggested an
> even bigger kernel change with pagetable_operations API.

OK.  I wasn't paying particularly close attention.  But my rant still
stands ;)

>  The new API
> was designed with an end goal of introduce /dev/hugetlb (as one of the
> feature, they might be thinking more).  What motivated me here is to
> point out that we can achieve the same goal of having a /dev/hugetlb
> with existing hugetlbfs infrastructure and the implementation is
> relatively straightforward.  What it also buys us is a bit more
> flexibility to the end user who wants to use the interface directly.

OK.

Why is it a "fuss" to do this with hugetlbfs files, btw?

Having read back through the thread, the only substantiation I can really
see is

  The pagetable_operations API opens up possibilities to do some
  additional (and completely sane) things.  For example, I have a patch
  that alters the character device code below to make use of a hugetlb
  ZERO_PAGE.  This eliminates almost all the up-front fault time, allowing
  pages to be COW'ed only when first written to.  We cannot do things like
  this with hugetlbfs anymore because we have a set of complex semantics to
  preserve.

Why is this actually a useful feature?

What does "complex semantics to preserve" mean?

I dunno.  I see a lot of code flying around, but comparatively little
effort to describe the actual problems which we're trying to solve.

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