linux-kernel - Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is not enough ram for it

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Date:	Tue, 08 Jan 2013 16:58:14 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	Yinghai Lu <yinghai@...nel.org>, Shuah Khan <shuahkhan@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>, Jan Kiszka <jan.kiszka@....de>,
	Jason Wessel <jason.wessel@...driver.com>,
	linux-kernel@...r.kernel.org, Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is not enough ram for it

Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> writes:

> 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@...nel.org> wrote:
>> > On Mon, Jan 7, 2013 at 7:13 PM, Eric W. Biederman <ebiederm@...ssion.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.

So instead we need to say?

+       if (no_iotlb_memory)                                                    
+               panic("Cannot allocate SWIOTLB buffer");
+                 

Which is just making the panic a little later than it used to be and
seems completely reasonable.

Eric
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