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Date: Tue, 20 May 2008 13:03:57 -0700
From: Arjan van de Ven <arjan@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Jens Axboe <jens.axboe@oracle.com>, 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 12:45:56 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> 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???

the sata_nv driver calls this from an invalid context ... and spews a
ton of warnings as a result... made me think this is a common mistake
to make.

I'd love to make it do your version instead, but I was afraid it would
trigger too often....