kern/78179: bus_dmamem_alloc() with BUS_DMA_NOWAIT can block

[Mark Tinguely]

The following reply was made to PR kern/78179; it has been noted by GNATS.

From: Mark Tinguely <tinguely at casselton.net>
To: PeterJeremy at optushome.com.au, tinguely at casselton.net
Cc: bug-followup at freebsd.org
Subject: Re: kern/78179: bus_dmamem_alloc() with BUS_DMA_NOWAIT can block
Date: Sat, 7 Jan 2006 09:15:08 -0600 (CST)

 Strange that the following added test in vm_contig_launder_page() did not
 trigger in the kernel_object case:
 
 	if (m->dirty) {
 +			/* Both paths use vm_pageout_flush() which forces 
 +			 * a syncronous putpage for the kernel_object.  
 +			 */
 +		if (flags & M_NOWAIT && object == kernel_object) {
 	...
 
 Anyway, I think it was concluded that when contigmalloc() is called from an
 interrupt handler, we cannot launder the pages at all. It simplifies
 the patch considerably. A couple things to consider:
 
 	1) it is wise for a driver to be requesting more than a page
 	   at interrupt time? It is a recipe for failure on busy machines.
 	   wouldn't it be better that the driver use some kind of UMA? This
 	   really a side issue and does not fix the error.
 
 	2) as mentioned before, many other callers to contigmalloc() that
 	   set NOWAIT, really expect laundering and waiting a brief amount
 	   of time. Many of these callers do not check for the allocation
 	   failure case. Most are allocating memory on device initialization
 	   and are not in their interrupt handlers. These issue have to
 	   be fixed or else we will actually be increasing the number of
 	   panics.
 
 --Mark Tinguely.