linux-kernel - Re: dmapool regression in next

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Date:   Thu, 6 Dec 2018 08:33:15 -0800
From:   Tony Lindgren <tony@...mide.com>
To:     Tony Battersby <tonyb@...ernetics.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        akpm@...ux-foundation.org, Stephen Rothwell <sfr@...b.auug.org.au>,
        john.garry@...wei.com, linux@...linux.org.uk,
        linux-kernel@...r.kernel.org, andy.shevchenko@...il.com,
        linux-omap@...r.kernel.org, hch@....de,
        linux-arm-kernel@...ts.infradead.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: dmapool regression in next

* Tony Battersby <tonyb@...ernetics.com> [181206 16:13]:
> On 12/6/18 10:51 AM, Robin Murphy wrote:
> >> Here is the prototype:
> >>
> >> void dma_pool_free(struct dma_pool *pool, void *vaddr, dma_addr_t dma);
> >>
> >> With the old code, the 'dma' value had to be correct for use with
> >> pool_find_page(), or else you would get an error.  If the 'vaddr' value
> >> was incorrect, it would corrupt the dmapool freelist, but you wouldn't
> >> get an error unless DMAPOOL_DEBUG was enabled.
> >>
> >> With my patch applied, 'vaddr' has to be correct for virt_to_page().  My
> >> code also checks that 'dma' is consistent with 'vaddr' even if
> >> DMAPOOL_DEBUG is disabled, since the check is fast and it will prevent
> >> problems like this in the future.
> > Unfortunately that logic has a fatal flaw - DMA pools are backed by 
> > dma_alloc_coherent(), and there is absolutely no guarantee that the 
> > memory dma_alloc_coherent() returns is backed by a struct page at all. 
> > Even if it is, there is still absolutely no guarantee that the vaddr 
> > value it returns is valid for virt_to_page() - on many systems it will 
> > be in vmalloc or some architecture-specific region of address space.
> >
> > The problem is not that these drivers are buggy (they're not - the arch 
> > code is returning a vmalloc()ed non-cacheable remap in the first place), 
> > it's that 26abe88e830d is fundamentally unworkable and needs reverting. 
> > Apparently the original patches managed not to catch my eye as something 
> > I needed to review, sorry about that :(
> >
> > Robin.
> >
> Thanks for the info; the inner workings of the vm system are a bit out
> of my area of expertise.  My first version of the patch series used a
> different method that didn't rely on virt_to_page(); I will go back to
> that version, clean it up, and resubmit when I have time.
> 
> Andrew, please revert all 9 patches.  I will resubmit the set when I
> have a workable solution.

OK sounds good to me. I can test the new set easily when available
if you Cc me on them.

Thanks,

Tony