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From: Takashi Iwai <tiwai@suse.de>
To: Keyon Jie <yang.jie@linux.intel.com>
Cc: "alsa-devel@alsa-project.org" <alsa-devel@alsa-project.org>,
	Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>,
	"Rajwa, Marcin" <marcin.rajwa@linux.intel.com>
Subject: Re: [alsa-devel] [PATCH] ALSA: pcm: fix buffer_bytes max constrained by preallocated bytes issue
Date: Fri, 17 Jan 2020 12:15:42 +0100	[thread overview]
Message-ID: <s5hmuam49lt.wl-tiwai@suse.de> (raw)
In-Reply-To: <476be658-59a6-e1f1-d7fc-51331966f27b@linux.intel.com>

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
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  reply	other threads:[~2020-01-17 11:16 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-16  4:53 [alsa-devel] [PATCH] ALSA: pcm: fix buffer_bytes max constrained by preallocated bytes issue Keyon Jie
2020-01-16  7:15 ` Takashi Iwai
2020-01-16  9:50   ` Keyon Jie
2020-01-16 10:27     ` Takashi Iwai
2020-01-16 11:25       ` Keyon Jie
2020-01-16 11:50         ` Takashi Iwai
2020-01-16 14:14           ` Jie, Yang
2020-01-16 15:31             ` Jie, Yang
2020-01-16 16:07               ` Takashi Iwai
2020-01-16 16:39               ` Pierre-Louis Bossart
2020-01-16 17:25                 ` Rajwa, Marcin
2020-01-16 17:40                   ` Pierre-Louis Bossart
2020-01-16 20:37                     ` Takashi Iwai
2020-01-17  5:30                       ` Keyon Jie
2020-01-17  7:57                         ` Takashi Iwai
2020-01-17 10:13                           ` Keyon Jie
2020-01-17 10:30                             ` Takashi Iwai
2020-01-17 10:56                               ` Keyon Jie
2020-01-17 11:15                                 ` Takashi Iwai [this message]
2020-01-17  5:37                     ` Keyon Jie
2020-01-17  8:00                       ` Takashi Iwai
2020-01-17 10:43                         ` Keyon Jie
2020-01-17 11:12                           ` Takashi Iwai
2020-01-19  3:52                             ` Keyon Jie
2020-01-19  7:09                               ` Takashi Iwai
2020-01-19  8:11                                 ` Keyon Jie
2020-01-19  9:04                                   ` Takashi Iwai
2020-01-19 10:14                                     ` Keyon Jie
2020-01-19 10:43                                       ` Takashi Iwai
2020-01-20  2:23                                         ` Keyon Jie
2020-01-16 15:45             ` Takashi Iwai

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