linux-kernel - Re: [PATCH] 2.6.26-rc: x86: pci-dma.c: use __GFP_NO_OOM instead of __GFP

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Message-Id: <1211963485.28138.14.camel@localhost.localdomain>
Date:	Wed, 28 May 2008 10:31:25 +0200
From:	Miquel van Smoorenburg <mikevs@...all.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Glauber Costa <gcosta@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...e.hu>, mikevs@...all.net
Subject: Re: [PATCH] 2.6.26-rc: x86: pci-dma.c: use __GFP_NO_OOM instead of
	__GFP_NORETRY

On Wed, 2008-05-28 at 04:47 +0200, Andi Kleen wrote:
> > So...  why not just remove the setting of __GFP_NORETRY?  Why is it
> > wrong to oom-kill things in this case?
> 
> When the 16MB zone overflows (which can be common in some workloads)
> calling the OOM killer is pretty useless because it has barely any 
> real user data [only exception would be the "only 16MB" case Alan
> mentioned]. Killing random processes in this case is bad. 
> 
> I think for 16MB __GFP_NORETRY is ok because there should be 
> nothing freeable in there so looping is useless. Only exception would be the 
> "only 16MB total" case again but I'm not sure 2.6 supports that at all
> on x86.
> 
> On the other hand d_a_c() does more allocations than just 16MB, especially
> on 64bit and the other zones need different strategies.

Okay, so how about this then ?

--- linux-2.6.26-rc4.orig/arch/x86/kernel/pci-dma.c	2008-05-26 20:08:11.000000000 +0200
+++ linux-2.6.26-rc4/arch/x86/kernel/pci-dma.c	2008-05-28 10:27:41.000000000 +0200
@@ -397,9 +397,6 @@
 	if (dev->dma_mask == NULL)
 		return NULL;
 
-	/* Don't invoke OOM killer */
-	gfp |= __GFP_NORETRY;
-
 #ifdef CONFIG_X86_64
 	/* Why <=? Even when the mask is smaller than 4GB it is often
 	   larger than 16MB and in this case we have a chance of
@@ -410,7 +407,9 @@
 #endif
 
  again:
-	page = dma_alloc_pages(dev, gfp, get_order(size));
+	/* Don't invoke OOM killer or retry in lower 16MB DMA zone */
+	page = dma_alloc_pages(dev,
+		(gfp & GFP_DMA) ? gfp | __GFP_NORETRY : gfp, get_order(size));
 	if (page == NULL)
 		return NULL;
 

Mike.

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