linux-kernel - Re: [PATCH v2] dma-direct: improve DMA_ATTR_NO_KERNEL

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Date:   Thu, 4 Nov 2021 20:18:33 +0800
From:   Walter Wu <walter-zh.wu@...iatek.com>
To:     Christoph Hellwig <hch@....de>
CC:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        "Ard Biesheuvel" <ardb@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <iommu@...ts.linux-foundation.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        wsd_upstream <wsd_upstream@...iatek.com>,
        <linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH v2] dma-direct: improve DMA_ATTR_NO_KERNEL_MAPPING

On Thu, 2021-11-04 at 09:53 +0100, Christoph Hellwig wrote:
> On Thu, Nov 04, 2021 at 10:32:21AM +0800, Walter Wu wrote:
> > diff --git a/include/linux/set_memory.h
> > b/include/linux/set_memory.h
> > index f36be5166c19..6c7d1683339c 100644
> > --- a/include/linux/set_memory.h
> > +++ b/include/linux/set_memory.h
> > @@ -7,11 +7,16 @@
> >  
> >  #ifdef CONFIG_ARCH_HAS_SET_MEMORY
> >  #include <asm/set_memory.h>
> > +
> > +#ifndef CONFIG_RODATA_FULL_DEFAULT_ENABLED
> 
> This is an arm64-specific symbol, and one that only controls a
> default.  I don't think it is suitable to key off stubs in common
> code.
> 

ok

> > +static inline int set_memory_valid(unsigned long addr, int
> > numpages, int enable) { return 0; }
> 
> Pleae avoid overly long lines.
> 
> > +		if (IS_ENABLED(CONFIG_RODATA_FULL_DEFAULT_ENABLED)) {
> > +			kaddr = (unsigned
> > long)phys_to_virt(dma_to_phys(dev, *dma_handle));
> 
> This can just use page_address.
> 
> > +			/* page remove kernel mapping for arm64 */
> > +			set_memory_valid(kaddr, size >> PAGE_SHIFT, 0);
> > +		}
> 
> But more importantly:  set_memory_valid only exists on arm64, this
> will break compile everywhere else.  And this API is complete crap.
> Passing kernel virtual addresses as unsigned long just sucks, and
> passing an integer argument for valid/non-valid also is a horrible
> API.
> 

Would you think __kernel_map_page() is ok?
Many arch support it, and only pass page and page number. but need to
depend CONFIG_DEBUG_PAGEALLOC.

Thanks.
Walter


> Not to mention the overly long line.  Same on the free side.