linux-kernel - Re: [PATCH v2] mm: dmapool: use provided gfp flags for all dma_alloc

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Date:	Tue, 22 Jan 2013 18:13:57 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Soeren Moch <smoch@....de>, Jason Cooper <jason@...edaemon.net>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Andrew Lunn <andrew@...n.ch>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...e.cz>,
	linux-mm@...ck.org, Kyungmin Park <kyungmin.park@...sung.com>,
	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	linaro-mm-sig@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
Subject: Re: [PATCH v2] mm: dmapool: use provided gfp flags for all dma_alloc_coherent() calls

On Monday 21 January 2013, Greg KH wrote:
> > 
> > I don't know a lot about USB, but I always assumed that this was not
> > a normal condition and that there are only a couple of URBs per endpoint
> > used at a time. Maybe Greg or someone else with a USB background can
> > shed some light on this.
> 
> There's no restriction on how many URBs a driver can have outstanding at
> once, and if you have a system with a lot of USB devices running at the
> same time, there could be lots of URBs in flight depending on the number
> of host controllers and devices and drivers being used.

Ok, thanks for clarifying that. I read some more of the em28xx driver,
and while it does have a bunch of URBs in flight, there are only five
audio and five video URBs that I see simultaneously being submitted,
and then resubmitted from their completion handlers. I think this
means that there should be 10 URBs active at any given time in this
driver, which does not explain why we get 256 allocations.

I also noticed that the initial submissions are all atomic but don't
need to, so it may be worth trying the patch below, which should also
help in low-memory situations. We could also try moving the resubmission
into a workqueue in order to let those be GFP_KERNEL, but I don't think
that will help.

	Arnd

diff --git a/drivers/media/usb/em28xx/em28xx-audio.c b/drivers/media/usb/em28xx/em28xx-audio.c
index 2fdb66e..8b789f4 100644
--- a/drivers/media/usb/em28xx/em28xx-audio.c
+++ b/drivers/media/usb/em28xx/em28xx-audio.c
@@ -177,12 +177,12 @@ static int em28xx_init_audio_isoc(struct em28xx *dev)
 		struct urb *urb;
 		int j, k;
 
-		dev->adev.transfer_buffer[i] = kmalloc(sb_size, GFP_ATOMIC);
+		dev->adev.transfer_buffer[i] = kmalloc(sb_size, GFP_KERNEL);
 		if (!dev->adev.transfer_buffer[i])
 			return -ENOMEM;
 
 		memset(dev->adev.transfer_buffer[i], 0x80, sb_size);
-		urb = usb_alloc_urb(EM28XX_NUM_AUDIO_PACKETS, GFP_ATOMIC);
+		urb = usb_alloc_urb(EM28XX_NUM_AUDIO_PACKETS, GFP_KERNEL);
 		if (!urb) {
 			em28xx_errdev("usb_alloc_urb failed!\n");
 			for (j = 0; j < i; j++) {
@@ -212,7 +212,7 @@ static int em28xx_init_audio_isoc(struct em28xx *dev)
 	}
 
 	for (i = 0; i < EM28XX_AUDIO_BUFS; i++) {
-		errCode = usb_submit_urb(dev->adev.urb[i], GFP_ATOMIC);
+		errCode = usb_submit_urb(dev->adev.urb[i], GFP_KERNEL);
 		if (errCode) {
 			em28xx_errdev("submit of audio urb failed\n");
 			em28xx_deinit_isoc_audio(dev);
diff --git a/drivers/media/usb/em28xx/em28xx-core.c b/drivers/media/usb/em28xx/em28xx-core.c
index bed07a6..c5a2c4b 100644
--- a/drivers/media/usb/em28xx/em28xx-core.c
+++ b/drivers/media/usb/em28xx/em28xx-core.c
@@ -1166,7 +1166,7 @@ int em28xx_init_isoc(struct em28xx *dev, enum em28xx_mode mode,
 
 	/* submit urbs and enables IRQ */
 	for (i = 0; i < isoc_bufs->num_bufs; i++) {
-		rc = usb_submit_urb(isoc_bufs->urb[i], GFP_ATOMIC);
+		rc = usb_submit_urb(isoc_bufs->urb[i], GFP_KERNEL);
 		if (rc) {
 			em28xx_err("submit of urb %i failed (error=%i)\n", i,
 				   rc);
--
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