[alsa-devel] [PATCH] ALSA: pcm: fix buffer_bytes max constrained by preallocated bytes issue

[Takashi Iwai]

On Fri, 17 Jan 2020 11:56:48 +0100,
Keyon Jie wrote:
> 
> On 2020/1/17 下午6:30, Takashi Iwai wrote:
> > On Fri, 17 Jan 2020 11:13:31 +0100,
> > Keyon Jie wrote:
> >>
> >>
> >>
> >> On 2020/1/17 下午3:57, Takashi Iwai wrote:
> >>> On Fri, 17 Jan 2020 06:30:18 +0100,
> >>> Keyon Jie wrote:
> >>>>
> >>>> On 2020/1/17 上午4:37, Takashi Iwai wrote:
> >>>>
> >>>> Hi Takashi, I get your concern here, but if we switch to use dma_max
> >>>> limit, we won't change the preallocated buffer, it will be still 64KB
> >>>> for each stream, user space can ask for re-allocate buffer for each
> >>>> stream up to 32MB, but those pinned and can't be swapped out ones are
> >>>> the 64KB preallocated ones only, am I wrong?
> >>>
> >>> No, in general, all sound hardware buffers are pinned.
> >>
> >> Sorry, I must have been wrong here, what I was focusing on is those
> >> allocated SG DMA buffers, I am not sure if they are those you called
> >> "hardware buffers" here.
> >>
> >> My understanding was like this:
> >>
> >> 1. in pcm_new() stage, the device PCM driver should call
> >> snd_pcm_lib_preallocate_pages()->
> >> 	snd_pcm_lib_preallocate_pages()->
> >> 		preallocate_pcm_pages()
> >> and then the substream->dma_buffer is initialized with the
> >> preallocated buffer.
> >>
> >> 2. in pcm_open() stage, the device PCM driver should call
> >> snd_pcm_lib_malloc_pages()->
> >> 	snd_dma_alloc_pages() //if we need to reallocate bigger
> >> buffer. *The substream->dma_buffer won't be freed, Takashi, this is
> >> what I thought you named "pinned" buffer.* And those reallocated
> >> bigger buffer via snd_dma_alloc_pages() will be freed at pcm_close()
> >> per my understanding?
> >
> > What I meant as "pinned" is that the pages are not swapped out by
> > swapper process like the user-space or anonymous pages.
> > So if you open all streams (say 16 streams) on a machine with 32MB
> > buffers, it'll cost a half GB.  And, we have no restriction about
> > which user may do it, so all normal users who have the access to the
> > sound device can consume a half GB kernel space pages easily.  For a
> > big server it's no problem, but for a small system, it's costing.
> 
> Understood, you are concerning about intentional attack from user
> space about memory consuming, you propose that normal user should be
> permitted to use the default 64KB only, if larger buffer required,
> please use proc fs expert mode, is my understanding correct?

Well, a normal user may want 1MB or 2MB buffer, and that's not too
bad.  So the most distros already set the larger preallocation for
HD-audio explicitly via CONFIG_SND_HDA_PREALLOC_SIZE without procfs
adjustment, I believe.  Then the system allows normal users buffers up
to the given size.


Takashi