linux-kernel - Re: [PATCH v3] dma-debug: fix incorrect pfn calculation

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Date:   Tue, 3 Oct 2017 09:07:52 +0200
From:   Christoph Hellwig <hch@....de>
To:     Miles Chen <miles.chen@...iatek.com>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        wsd_upstream@...iatek.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH v3] dma-debug: fix incorrect pfn calculation

On Mon, Oct 02, 2017 at 06:30:41PM +0800, Miles Chen wrote:
> ARCHs like metag and xtensa define their mappings (non-vmalloc and
> non-linear) for dma allocation.

metag basically is a reimplementation of the vmalloc map mechanism
that should be easy to consolidate into the common one :(  xtensa
has a weird remapping into a different segment, something that I
vaguely remember mips used to support as well.

> These mapping types are architecture-dependent and should not be used
> outside arch folders. So it is hard to check the mappings and convert
> a virtual address to a correct pfn in lib/dam-debug.c
> 
> How about recording only vmalloc (by is_vmalloc_addr()) and linear
> address (by virt_addr_valid()) in lib/dma-debug? Since current 
> implementation is not correct for those ARCHs.
> 
> if (!is_vmalloc_addr(addr) && !virt_addr_valid(addr))
>     return;
> 
> or

This looks like a good start, although I'm not sure I'd trust
virt_addr_valid on every little arch.  In the worse case we'll
have to exclude offenders from supporting dma debug, so let's go
with that version.

> > > > +	entry->pfn	 = is_vmalloc_addr(virt) ? vmalloc_to_pfn(virt) :
> > > > +						page_to_pfn(virt_to_page(virt));
> > 
> > Please use normal if/else conditionsals:
> 
> Is this for better readability? I'll send another patch for this.

Yes.