linux-kernel - Re: 3.14-mw regression: rtl8169 WARNING: DMA-API: exceeded 7 overlapping mappings of pfn 55ebe

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Date:	Thu, 13 Feb 2014 12:14:47 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Sander Eikelenboom <linux@...elenboom.it>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Wei Liu <wei.liu2@...rix.com>,
	Francois Romieu <romieu@...zoreil.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Russell King <rmk+kernel@....linux.org.uk>
Subject: Re: 3.14-mw regression: rtl8169 WARNING: DMA-API: exceeded 7
 overlapping mappings of pfn 55ebe

On Tue, 2014-02-11 at 20:17 -0800, Eric Dumazet wrote:
> On Tue, 2014-02-11 at 18:07 -0800, Dan Williams wrote:
> 
> > The overlap granularity is too large.  Multiple dma_map_single
> > mappings are allowed to a given page as long as they don't collide on
> > the same cache line.
> > 
> 
> I am not sure why you try number of mappings of a page.
> 
> Try launching 100 concurrent netperf -t TCP_SENFILE
> 
> Same page might be mapped more than 100 times, more than 10000 times in
> some cases.

Thanks for that test case.

I updated the fix patch with the following.

diff --git a/lib/dma-debug.c b/lib/dma-debug.c
index 42b12740940b..611010df1e9c 100644
--- a/lib/dma-debug.c
+++ b/lib/dma-debug.c
@@ -513,6 +513,13 @@ static int active_cln_insert(struct dma_debug_entry *entry)
        unsigned long flags;
        int rc;
 
+       /* If the device is not writing memory then we don't have any
+        * concerns about the cpu consuming stale data.  This mitigates
+        * legitimate usages of overlapping mappings.
+        */
+       if (entry->direction == DMA_TO_DEVICE)
+               return 0;
+
        spin_lock_irqsave(&radix_lock, flags);
        rc = radix_tree_insert(&dma_active_cacheline, to_cln(entry), entry);
        if (rc == -EEXIST)
@@ -526,6 +533,10 @@ static void active_cln_remove(struct dma_debug_entry *entry)
 {
        unsigned long flags;
 
+       /* ...mirror the insert case */
+       if (entry->direction == DMA_TO_DEVICE)
+               return;
+
        spin_lock_irqsave(&radix_lock, flags);
        /* since we are counting overlaps the final put of the
         * cacheline will occur when the overlap count is 0.


Sander, barring a negative test result from you I'll send the attached
patch to Andrew.

--
Dan


View attachment "fix-dma-debug-overlap-v2.patch" of type "text/x-patch" (10507 bytes)