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 16:04:52 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Shuah Khan <shuahkhan@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.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

Yinghai Lu <yinghai@...nel.org> writes:

> 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.
>

It looks like the right direction.   Certainly enough to test and see if
the code will work.

I don't see the point of adding a nopanic case to the swiotlb
initialization.  That just looks like unnecessary complications.
Certainly a nopanic case implemented by passing a nopanic parameter
looks like the wrong way to go.  At most you want to return an error
code and do:

swiotlb_init()
{
	if (swiotlb_init_with_default_size() == -ENOMEM)
		panic("Cannot allocate SWIOTLB buffer");
}

The page freeing in swiotlb_init_with_tbl appears to be in the wrong
function.  I suggest looking at swiotlb_late_init which apparently is
allowed to fail for some ideas.

> looks like it need more change of lines.

The size of the change matters less than how clean and maintainable the
result is.  If done carefully I expect you can have net fewer lines but
not needing to handle the case when the swiotlb apis are unavailable.

Eric
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