linux-kernel - Re: [PATCH] CMA: Don't return a valid cma for non-cma dev

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Date:	Fri, 31 Jul 2015 02:51:11 +0000
From:	"Tang, Feng" <feng.tang@...el.com>
To:	"mina86@...a86.com" <mina86@...a86.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
	"kyungmin.park@...sung.com" <kyungmin.park@...sung.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"iamjoonsoo.kim@....com" <iamjoonsoo.kim@....com>
Subject: Re: [PATCH] CMA: Don't return a valid cma for non-cma dev

Hi Michal Nazarewicz,

Thanks for the review.

On Thu, 2015-07-30 at 15:59 +0200, Michal Nazarewicz wrote:
> On Thu, Jul 30 2015, Feng Tang wrote:
> > When system(one x86 soc) boot, we saw many normal dma allocation requests
> > goes to cma area. The call chain is
> > 	dma_generic_alloc_coherent
> > 	    dma_alloc_from_contiguous	-- arch/x86/kernel/pci-dma.c
> > 	        cma_alloc(dev_get_cma_area(dev), count, align)
> >
> > Current dev_get_cma_area() will return a valid "cma" anyway. Then all
> > these requests will be taken as valid cma request, and get pages from
> > cma area, which has 2 problems:
> > 1. make the cma area fragmented
> > 2. confuse the cma reservation, usually cma memory size is set according
> >    to the expectation of system scenario, these unexpected requests
> >    will affect the designed cma usage.
> >
> > So this patch will enforce the judgement, and only return valid "cma"
> > for real cma user, thus make normal user like IO device driver not
> > abuse cma reserved region.
> 
> Just don’t set dma_contiguous_default_area.  This patch defeats the
> purpose of a *default* area.

Yes! This is exactly what I tried when I first saw this problem, but
this failed as there 2 places inside drivers/base/dma-contiguous.c
which set the dma_contiguous_default_area

1) 
void __init dma_contiguous_reserve(phys_addr_t limit)
{
    ....
    dma_contiguous_reserve_area(selected_size, selected_base,
                                            selected_limit,
                                   &dma_contiguous_default_area,
                                            fixed);
     ....
}

2) 
static int __init rmem_cma_setup(struct reserved_mem *rmem)
{
    ...
    if (of_get_flat_dt_prop(node, "linux,cma-default", NULL))
                dma_contiguous_set_default(cma);
    ...
}

Are you suggesting me to remove them?

Thanks,
Feng