linux-kernel - Re: regression breaks lowmem reserved RAM

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Message-ID: <20080401124553.GM12346@kernel.dk>
Date:	Tue, 1 Apr 2008 14:45:54 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrea Arcangeli <andrea@...ranet.com>
Cc:	Andrew Morton <akpm@...l.org>, Yang Shi <yang.shi@...driver.com>,
	linux-kernel@...r.kernel.org, kvm-devel@...ts.sourceforge.net
Subject: Re: regression breaks lowmem reserved RAM

On Tue, Apr 01 2008, Andrea Arcangeli wrote:
> Looking a bit closer into this regression the reason this can't be
> right is that dma_addr common default is BLK_BOUNCE_HIGH and most
> machines have less than 4G. So if you do:
> 
>     if (b_pfn <= (min_t(u64, 0xffffffff, BLK_BOUNCE_HIGH) >> PAGE_SHIFT))
> 	dma = 1
> 
> that will translate to:
> 
>      if (BLK_BOUNCE_HIGH <= BLK_BOUNCE_HIGH)
>      	dma = 1
> 
> So for 99% of hardware this will trigger unnecessary GFP_DMA
> allocations and isa pooling operations.
> 
> Also note how the 32bit code still does b_pfn < blk_max_low_pfn.
> 
> I guess this is what you were looking after. I didn't verify but as
> far as I can tell, this will stop the regression with isa dma
> operations at boot for 99% of blkdev/memory combinations out there and
> I guess this fixes the setups with >4G of ram and 32bit pci cards as
> well (this also retains symmetry with the 32bit code).
> 
> Signed-off-by: Andrea Arcangeli <andrea@...ranet.com>

Thanks Andrea, this looks much saner!

-- 
Jens Axboe

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