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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 11:30:02 +0100
Message-ID: <s5h1rry5qad.wl-tiwai@suse.de> (raw)
In-Reply-To: <d6cee7a3-bbd4-0706-fcc2-96b403f6c58e@linux.intel.com>

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.


Takashi
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Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-16  4:53 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 [this message]
2020-01-17 10:56                               ` Keyon Jie
2020-01-17 11:15                                 ` Takashi Iwai
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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