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

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Date:	Thu, 23 Oct 2014 19:14:15 -0400
From:	Dan Streetman <ddstreet@...e.org>
To:	Seth Jennings <sjennings@...iantweb.net>
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 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.


>
> 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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