Date:	Fri, 08 Sep 2006 15:06:28 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Andrew Morton <akpm@...l.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/8] Avoiding fragmentation with subzone groupings v25

On Fri, 2006-09-08 at 09:36 +0100, Mel Gorman wrote:
> On Thu, 7 Sep 2006, Andrew Morton wrote:
> 
> > On Thu,  7 Sep 2006 20:03:42 +0100 (IST)
> > Mel Gorman <mel@....ul.ie> wrote:
> >
> >> When a page is allocated, the page-flags
> >> are updated with a value indicating it's type of reclaimability so that it
> >> is placed on the correct list on free.
> >
> > We're getting awful tight on page-flags.
> >
> 
> Yeah, I know :(
> 
> > Would it be possible to avoid adding the flag?  Say, have a per-zone bitmap
> > of size (zone->present_pages/(1<<MAX_ORDER)) bits, then do a lookup in
> > there to work out whether a particular page is within a MAX_ORDER clump of
> > easy-reclaimable pages?
> >
> 
> An early version of the patches created such a bitmap and it was heavily 
> resisted for two reasons. It put more pressure on the cache and it needed 
> to be resized during hot-add and hot-remove. It was the latter issue 
> people had more problems with. However, I can reimplement it if people 
> want to take a look. As I see it currently, there are five choices that 
> could be taken to avoid using an additional pageflag
> 
> 1. Re-use existing page flags. This is what I currently do in a later
>     patch for the software suspend flags
>     pros: Straight-forward implementation, appears to use no additional flags
>     cons: When swsusp stops using the flags, anti-frag takes them right back
>           Makes anti-frag mutually exclusive with swsusp
> 
> 2. Create a per-zone bitmap for every MAX_ORDER block
>     pros: Straight-forward implementation initially
>     cons: Needs resizing during hotadd which could get complicated
>           Bit more cache pressure
> 
> 3. Use the low two bits of page->lru
>     pros: Uses existing struct page field
>     cons: It's a bit funky looking
> 
> 4. Use the page->flags of the struct page backing the pages used
>     for the memmap.
>     pros: Similar to the bitmap idea except with less hotadd problems
>     cons: Bit more cache pressure
> 
> 5. Add an additional field page->hintsflags used for non-critical flags.
>     There are patches out there like guest page hinting that want to
>     consume flags but not for any vital purpose and usually for machines
>     that have ample amounts of memory. For these features, add an
>     additional page->hintsflags
>     pros: Straight-forward to implement
>     cons: Increses struct page size for some kernel features.
> 
> I am leaning towards option 3 because it uses no additional memory but I'm 
> not sure how people feel about using pointer magic like this.
> 
> Any opinions?

If, as you stated in a previous mail, you'd like to have flags per
MAX_ORDER block, you'd already have to suffer the extra cache pressure.
In that case I vote for 4.

Otherwise 3 sounds doable, we already hide PAGE_MAPPING_ANON in a
pointer, so hiding flags is not new to struct page. It's just a question
of how good the implementation will look, I hope you'll not have to
visit all the list ops.

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