linux-kernel - Re: [RFC PATCH 0/9] mm/zbud: support highmem pages

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Date:	Fri, 24 Oct 2014 10:27:05 -0500
From:	Seth Jennings <sjennings@...iantweb.net>
To:	Dan Streetman <ddstreet@...e.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Heesub Shin <heesub.shin@...sung.com>,
	Nitin Gupta <ngupta@...are.org>, Linux-MM <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Sunae Seo <sunae.seo@...sung.com>
Subject: Re: [RFC PATCH 0/9] mm/zbud: support highmem pages

On Thu, Oct 23, 2014 at 07:14:15PM -0400, Dan Streetman wrote:
> On Tue, Oct 14, 2014 at 7:59 AM, Heesub Shin <heesub.shin@...sung.com> wrote:
> > zbud is a memory allocator for storing compressed data pages. It keeps
> > two data objects of arbitrary size on a single page. This simple design
> > provides very deterministic behavior on reclamation, which is one of
> > reasons why zswap selected zbud as a default allocator over zsmalloc.
> >
> > Unlike zsmalloc, however, zbud does not support highmem. This is
> > problomatic especially on 32-bit machines having relatively small
> > lowmem. Compressing anonymous pages from highmem and storing them into
> > lowmem could eat up lowmem spaces.
> >
> > This limitation is due to the fact that zbud manages its internal data
> > structures on zbud_header which is kept in the head of zbud_page. For
> > example, zbud_pages are tracked by several lists and have some status
> > information, which are being referenced at any time by the kernel. Thus,
> > zbud_pages should be allocated on a memory region directly mapped,
> > lowmem.
> >
> > After some digging out, I found that internal data structures of zbud
> > can be kept in the struct page, the same way as zsmalloc does. So, this
> > series moves out all fields in zbud_header to struct page. Though it
> > alters quite a lot, it does not add any functional differences except
> > highmem support. I am afraid that this kind of modification abusing
> > several fields in struct page would be ok.
> 
> Seth, have you had a chance to review this yet?  I'm going to try to
> take a look at it next week if you haven't yet.  Letting zbud use
> highmem would be a good thing.

I have looked at it, and it looks sound to me.  I seem to remember
having a comment on something, but I'll have to look back over
it. Haven't tested it yet.

Seth

> 
> 
> >
> > Heesub Shin (9):
> >   mm/zbud: tidy up a bit
> >   mm/zbud: remove buddied list from zbud_pool
> >   mm/zbud: remove lru from zbud_header
> >   mm/zbud: remove first|last_chunks from zbud_header
> >   mm/zbud: encode zbud handle using struct page
> >   mm/zbud: remove list_head for buddied list from zbud_header
> >   mm/zbud: drop zbud_header
> >   mm/zbud: allow clients to use highmem pages
> >   mm/zswap: use highmem pages for compressed pool
> >
> >  mm/zbud.c  | 244 ++++++++++++++++++++++++++++++-------------------------------
> >  mm/zswap.c |   4 +-
> >  2 files changed, 121 insertions(+), 127 deletions(-)
> >
> > --
> > 1.9.1
> >
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