linux-kernel - Re: [PATCH v4] dma-debug: fix incorrect pfn calculation

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Date:   Thu, 16 Nov 2017 18:45:01 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Miles Chen <miles.chen@...iatek.com>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <wsd_upstream@...iatek.com>, <linux-mediatek@...ts.infradead.org>,
        <iommu@...ts.linux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v4] dma-debug: fix incorrect pfn calculation

On Fri, 17 Nov 2017 09:23:18 +0800 Miles Chen <miles.chen@...iatek.com> wrote:

> On Thu, 2017-11-16 at 16:13 -0800, Andrew Morton wrote:
> > On Fri, 17 Nov 2017 06:56:12 +0800 <miles.chen@...iatek.com> wrote:
> > 
> > > From: Miles Chen <miles.chen@...iatek.com>
> > > 
> > > dma-debug reports the following warning:
> > > 
> > > [name:panic&]WARNING: CPU: 3 PID: 298 at kernel-4.4/lib/dma-debug.c:604
> > > debug _dma_assert_idle+0x1a8/0x230()
> > > DMA-API: cpu touching an active dma mapped cacheline [cln=0x00000882300]
> > > CPU: 3 PID: 298 Comm: vold Tainted: G        W  O    4.4.22+ #1
> > > Hardware name: MT6739 (DT)
> > > Call trace:
> > > [<ffffff800808acd0>] dump_backtrace+0x0/0x1d4
> > > [<ffffff800808affc>] show_stack+0x14/0x1c
> > > [<ffffff800838019c>] dump_stack+0xa8/0xe0
> > > [<ffffff80080a0594>] warn_slowpath_common+0xf4/0x11c
> > > [<ffffff80080a061c>] warn_slowpath_fmt+0x60/0x80
> > > [<ffffff80083afe24>] debug_dma_assert_idle+0x1a8/0x230
> > > [<ffffff80081dca9c>] wp_page_copy.isra.96+0x118/0x520
> > > [<ffffff80081de114>] do_wp_page+0x4fc/0x534
> > > [<ffffff80081e0a14>] handle_mm_fault+0xd4c/0x1310
> > > [<ffffff8008098798>] do_page_fault+0x1c8/0x394
> > > [<ffffff800808231c>] do_mem_abort+0x50/0xec
> > > 
> > > I found that debug_dma_alloc_coherent() and debug_dma_free_coherent()
> > > assume that dma_alloc_coherent() always returns a linear address.  However
> > > it's possible that dma_alloc_coherent() returns a non-linear address.  In
> > > this case, page_to_pfn(virt_to_page(virt)) will return an incorrect pfn.
> > > If the pfn is valid and mapped as a COW page, we will hit the warning when
> > > doing wp_page_copy().
> > > 
> > > Fix this by calculating pfn for linear and non-linear addresses.
> > > 
> > 
> > It's a shame you didn't Cc Christoph, who was the sole reviewer of the
> > earlier version.
> > 
> > And it's a shame you didn't capture the result of that review
> > discussion in the v3 changelog.
> > 
> > And it's a shame that you didn't describe how this patch differs from
> > earlier versions.
> 
> 
> I am truly sorry about this. I was not sure if I can submit a patch
> based on a linux-next patch, so I submit a new patch based on the latest
> mainline kernel again.
> 
> I know how to do this now. I will do it correctly next time.
> 
> Is there anyway to fix this? (send another patch with v3 discussion and
> the difference from earlier versions to the commit message).

A complete resend is perfectly OK - I will handle the changelog
modifications, etc.

My point is that the Cc: line was incomplete and that the changelog is
missing information, as described above.