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Date: Tue, 20 May 2008 12:45:56 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Jens Axboe <jens.axboe@oracle.com>
Cc: arjan@infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] block: blk_queue_bounce_limits can actually sleep
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On Tue, 20 May 2008 21:29:59 +0200
Jens Axboe <jens.axboe@oracle.com> wrote:

> On Mon, May 19 2008, Arjan van de Ven wrote:
> > From: Arjan van de Ven <arjan@linux.intel.com>
> > Subject: [PATCH] block: blk_queue_bounce_limits can actually sleep
> > 
> > blk_queue_bounce_limit can call init_emergency_isa_pool, which
> > does sleeping allocations... document it as such by adding
> > might_sleep() to the driver
> 
> Isn't that superflous, as mempool_create() -> kmalloc(..., __GFP_WAIT)
> ends up spewing that warning anyway?

It's largely superfluous given the way in which Arjan implemented it.

One situation which we regularly hit is:

foo()
{
	...
	if (some_unlikely_condition())
		do_something_which_sleeps();
	...
}

and then we go and call that code under spinlock and ship it out, when
of course a handful of testers hit the unlikely condition.

The solution to that is to add a might_sleep() _outside_ the test of
some_unlikely_condition().  ie:

--- a/block/blk-settings.c~a
+++ a/block/blk-settings.c
@@ -140,6 +140,8 @@ void blk_queue_bounce_limit(struct reque
 	unsigned long b_pfn = dma_addr >> PAGE_SHIFT;
 	int dma = 0;
 
+	might_sleep();
+
 	q->bounce_gfp = GFP_NOIO;
 #if BITS_PER_LONG == 64
 	/* Assume anything <= 4GB can be handled by IOMMU.
_

but it's all vague and waffly because Arjan forgot to tell us why he's
bothering to patch this code at all???