linux-kernel - RE: [PATCH 2/2] zram: clean up handle

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Date:	Thu, 7 Jun 2012 14:20:40 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Nitin Gupta <ngupta@...are.org>
Cc:	Minchan Kim <minchan@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Konrad Wilk <konrad.wilk@...cle.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>
Subject: RE: [PATCH 2/2] zram: clean up handle

> From: Nitin Gupta [mailto:ngupta@...are.org]
> > Nitin, can zsmalloc allow full page allocation by assigning
> > an actual physical pageframe (which is what zram does now)?
> > Or will it allocate PAGE_SIZE bytes which zsmalloc will allocate
> > crossing a page boundary which, presumably, will have much worse
> > impact on page allocator availability when these pages are
> > "reclaimed" via your swap notify callback.
> 
> zsmalloc does not add any object headers, so when allocating PAGE_SIZE
> you get a separate page from as if you did alloc_page(). So, it does not
> span page boundaries.
> 
> > Though this may be rare across all workloads, it may turn out
> > to be very common for certain workloads (e.g. if the workload
> > has many dirty anonymous pages that are already compressed
> > by userland).
> >
> > It may not be worth cleaning up the code if it causes
> > performance issues with this case.
> >
> > And anyway can zsmalloc handle and identify to the caller pages
> > that are both compressed and "native" (uncompressed)?  It
> > certainly has to handle both if you remove ZRAM_UNCOMPRESSED
> > as compressing some pages actually results in more than
> > PAGE_SIZE bytes.  So you need to record somewhere that
> > this "compressed page" is special and that must somehow
> > be communicated to the caller of your "get" routine.
> >
> > (Just trying to save Minchan from removing all that code but
> > then needing to add it back again.)
> 
> zsmalloc cannot identify compressed vs uncompressed pages. However, in
> zram, we can tell if the page is uncompressed using table[i]->size which
> is set to PAGE_SIZE for uncompressed pages.   Pages that compress to
> more than PAGE_SIZE (i.e. expand on compression) are stored
> as-is/uncompressed and thus will have size field set to PAGE_SIZE.
> 
> Thus, we do not require ZRAM_UNCOMPRESSED flag when using zsmalloc for
> both compressed and uncompressed pages.

Good to know.  Nice work in zsmalloc and zram!

Dan
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