Re: [PATCH] mm: cma: allocate pages from CMA if NR_FREE_PAGES approaches low water mark

[Minchan Kim <minchan@kernel.org>]

On Wed, Nov 21, 2012 at 02:07:04PM +0100, Michal Nazarewicz wrote:
> On Wed, Nov 21 2012, Minchan Kim wrote:
> > So your concern is that too many free pages in MIGRATE_CMA when OOM happens
> > is odd? It's natural with considering CMA design which kernel never fallback
> > non-movable page allocation to CMA area. I guess it's not a your concern.
> >
> > Let's think below extreme cases.
> >
> > = Before =
> >
> > * 1000M DRAM system.
> > * 400M kernel used pages.
> > * 300M movable used pages.
> > * 300M cma freed pages.
> >
> > 1. kernel want to request 400M non-movable memory, additionally.
> > 2. VM start to reclaim 300M movable pages.
> > 3. But it's not enough to meet 400M request.
> > 4. go to OOM. (It's natural)
> >
> > = After(with your patch) =
> >
> > * 1000M DRAM system.
> > * 400M kernel used pages.
> > * 300M movable *freed* pages.
> > * 300M cma used pages(by your patch, I simplified your concept)
> >
> > 1. kernel want to request 400M non-movable memory.
> > 2. 300M movable freed pages isn't enough to meet 400M request.
> > 3. Also, there is no point to reclaim CMA pages for non-movable allocation.
> > 4. go to OOM. (It's natural)
> >
> > There is no difference between before and after in allocation POV.
> > Let's think another example.
> >
> > = Before =
> >
> > * 1000M DRAM system.
> > * 400M kernel used pages.
> > * 300M movable used pages.
> > * 300M cma freed pages.
> >
> > 1. kernel want to request 300M non-movable memory.
> > 2. VM start to reclaim 300M movable pages.
> > 3. It's enough to meet 300M request.
> > 4. happy end
> >
> > = After(with your patch) =
> >
> > * 1000M DRAM system.
> > * 400M kernel used pages.
> > * 300M movable *freed* pages.
> > * 300M cma used pages(by your patch, I simplified your concept)
> >
> > 1. kernel want to request 300M non-movable memory.
> > 2. 300M movable freed pages is enough to meet 300M request.
> > 3. happy end.
> >
> > There is no difference in allocation POV, too.
> 
> The difference thou is that before 30% of memory is wasted (ie. free),
> whereas after all memory is used.  The main point of CMA is to make the
> memory useful if devices are not using it.  Having it not allocated is
> defeating that purpose.

I think it's not a waste because if reclaimed movable pages is working set,
they are soon reloaded to migrate_cma in this time.

> 
> -- 
> Best regards,                                         _     _
> .o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
> ..o | Computer Science,  MichaA? a??mina86a?? Nazarewicz    (o o)
> ooo +----<email/xmpp: mpn@google.com>--------------ooO--(_)--Ooo--





-- 
Kind Regards,
Minchan Kim

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