Re: [PATCH] mm: cma: fix accounting of CMA pages placed in high memory

[Simon Jeons]

On 02/20/2013 10:57 AM, Kyungmin Park wrote:
> On Tue, Feb 19, 2013 at 10:27 PM, Simon Jeons <simon.jeons@xxxxxxxxx> wrote:
> > On 02/05/2013 03:10 PM, Marek Szyprowski wrote:
> > > Hello,
> > >
> > > On 2/5/2013 12:34 AM, Minchan Kim wrote:
> > > > On Mon, Feb 04, 2013 at 11:27:05AM +0100, Marek Szyprowski wrote:
> > > > > The total number of low memory pages is determined as
> > > > > totalram_pages - totalhigh_pages, so without this patch all CMA
> > > > > pageblocks placed in highmem were accounted to low memory.
> > > > So what's the end user effect? With the effect, we have to decide
> > > > routing it on stable.
> > > >
> > > > > Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> > > > > ---
> > > > >   mm/page_alloc.c |    4 ++++
> > > > >   1 file changed, 4 insertions(+)
> > > > >
> > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > > index f5bab0a..6415d93 100644
> > > > > --- a/mm/page_alloc.c
> > > > > +++ b/mm/page_alloc.c
> > > > > @@ -773,6 +773,10 @@ void __init init_cma_reserved_pageblock(struct
> > > > > page *page)
> > > > >       set_pageblock_migratetype(page, MIGRATE_CMA);
> > > > >       __free_pages(page, pageblock_order);
> > > > >       totalram_pages += pageblock_nr_pages;
> > > > > +#ifdef CONFIG_HIGHMEM
> > > > We don't need #ifdef/#endif.
> > >
> > > #ifdef is required to let this code compile when highmem is not enabled,
> > > becuase totalhigh_pages is defined as 0, see include/linux/highmem.h
> > Hi Marek,
> >
> > 1) Why can support CMA regions placed in highmem?
> Some vendors use reserved memory at highmem, and it's hard to modify
> to use lowmem, so just CMA can support highmem and no need to adjust
> address used at reserved memory.
> > CMA is for dma buffer, correct? Then how can old dma device access highmem?
> What's the "old dma device"? To support it, we also modify

I mean dma devices which should still use bounce buffer, then these 
devices will use bounce buffer to access CMA region in highmem or ...?

> arch/arm/mm/dma-mapping.c for handling highmem address.
>
> > 2) Why there is no totalhigh_pages variable define in the case of config
> > highmem?
> I don't know. it's just defined in case of highmem. that's reason to
> use #ifdef/endif. we don't know hisotrical reason.
>
> Thank you,
> Kyungmin Park
> > > > > +    if (PageHighMem(page))
> > > > > +        totalhigh_pages += pageblock_nr_pages;
> > > > > +#endif
> > > > >   }
> > > > >   #endif
> > >
> > > Best regards
> >
> > --
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