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Date: Tue, 8 Jan 2013 19:43:41 -0500
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To: Yinghai Lu <yinghai@kernel.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
        Shuah Khan <shuahkhan@gmail.com>, Thomas Gleixner <tglx@linutronix.de>,
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Subject: Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is not
 enough ram for it
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On Tue, Jan 08, 2013 at 03:40:11PM -0800, Yinghai Lu wrote:
> On Mon, Jan 7, 2013 at 7:50 PM, Yinghai Lu <yinghai@kernel.org> wrote:
> > On Mon, Jan 7, 2013 at 7:13 PM, Eric W. Biederman <ebiederm@xmission.com> wrote:
> >> I meant we should detect failure to allocate bounce buffers in in
> >> swiotlb_init() instead of panicing.
> >>
> >> I meant swiotlb_map_single() should either panic or simply fail.
> >>
> >> If I have read lib/swiotlb.c correctly the only place we allocate a
> >> bounce buffer is in swiotlb_map_single.  If there are more places we can
> >> allocate bounce buffers those need to be handled as well.
> >
> > ok, will give it a try.
> 
> please check if you are ok with attached.
> 
> looks like it need more change of lines.

The swiotlb_full check I don't believe is neccessary. You won't ever get
to that unless swiotlb_map_page has at least provided a bounce buffer.
And if the swiotlb_map_page does not have a bounce buffer it will exit
with:

+       if (no_iotlb_memory)                                                    
+               return SWIOTLB_MAP_ERROR;                                       
+                 

which is dangerous. That is b/c there are drivers that don't use the
dma_mapping_error check (so check the bus address after calling
pci_map_*). This means they would try to do DMA on 0xffffffff (yikes!).

That is reason the failback (v_overflow_buffer) is still in
usage - b/c we have drivers that might just do this and this is the last
resort for them. And until those drivers are fixed - we _need_ this
fallback to work.