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

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Date:   Thu, 17 Dec 2020 12:06:47 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Lars-Peter Clausen <lars@...afoo.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 Thu, 17 Dec 2020 11:59:23 +0100,
Lars-Peter Clausen wrote:
> 
> 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.

IIRC, we used GFP_ZERO in the past for the normal page allocations,
but this was dropped as it's no longer supported or so.

Also, we clear out the PCM buffer in hw_params call, but this is for
the requested size, not the actual allocated size, hence the padding
bytes will remain uncleared.

So I believe it's safer to add an extra memset() like my test patch.


Takashi