linux-kernel - Re: [PATCH v1 ] ALSA: core: memalloc: add page alignment for iram

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Message-ID: <70074f62-954a-9b40-ab4a-cb438925060c@metafoo.de>
Date:   Thu, 17 Dec 2020 11:59:23 +0100
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     alsa-devel@...a-project.org, gustavoars@...nel.org,
        linux-kernel@...r.kernel.org, shengjiu.wang@....com,
        pierre-louis.bossart@...ux.intel.com, tiwai@...e.com,
        xiang@...nel.org, Robin Gong <yibin.gong@....com>,
        akpm@...ux-foundation.org
Subject: Re: [PATCH v1 ] ALSA: core: memalloc: add page alignment for iram

On 12/17/20 10:55 AM, Takashi Iwai wrote:
> On Thu, 17 Dec 2020 10:43:45 +0100,
> Lars-Peter Clausen wrote:
>> On 12/17/20 5:15 PM, Robin Gong wrote:
>>> Since mmap for userspace is based on page alignment, add page alignment
>>> for iram alloc from pool, otherwise, some good data located in the same
>>> page of dmab->area maybe touched wrongly by userspace like pulseaudio.
>>>
>> I wonder, do we also have to align size to be a multiple of PAGE_SIZE
>> to avoid leaking unrelated data?
> Hm, a good question.  Basically the PCM buffer size itself shouldn't
> be influenced by that (i.e. no hw-constraint or such is needed), but
> the padding should be cleared indeed.  I somehow left those to the
> allocator side, but maybe it's safer to clear the whole buffer in
> sound/core/memalloc.c commonly.

What I meant was that most of the APIs that we use to allocate memory 
work on a PAGE_SIZE granularity. I.e. if you request a buffer that where 
the size is not a multiple of PAGE_SIZE internally they will still 
allocate a buffer that is a multiple of PAGE_SIZE and mark the unused 
bytes as reserved.

But I believe that is not the case gen_pool_dma_alloc(). It will happily 
allocate those extra bytes to some other allocation request.

That we need to zero out the reserved bytes even for those other APIs is 
a very good additional point!

I looked at this a few years ago and I'm pretty sure that we cleared out 
the allocated area, but I can't find that anymore in the current code. 
Which is not so great I guess.