linux-kernel - Re: regression breaks lowmem reserved RAM

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Date:	Tue, 01 Apr 2008 19:27:02 +0800
From:	yshi <yang.shi@...driver.com>
To:	Andrea Arcangeli <andrea@...ranet.com>
CC:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	kvm-devel@...ts.sourceforge.net, Jens Axboe <axboe@...nel.dk>
Subject: Re: regression breaks lowmem reserved RAM

Hi Andrea,

This patch works well on my machine. Thanks.

Best Regards,
Yang

Andrea Arcangeli ??:
> 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>
>
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index 1344a0e..5713f7e 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -140,7 +140,7 @@ void blk_queue_bounce_limit(struct request_queue *q, u64 dma_addr)
>  	/* Assume anything <= 4GB can be handled by IOMMU.
>  	   Actually some IOMMUs can handle everything, but I don't
>  	   know of a way to test this here. */
> -	if (b_pfn <= (min_t(u64, 0xffffffff, BLK_BOUNCE_HIGH) >> PAGE_SHIFT))
> +	if (b_pfn < (min_t(u64, 0x100000000UL, BLK_BOUNCE_HIGH) >> PAGE_SHIFT))
>  		dma = 1;
>  	q->bounce_pfn = max_low_pfn;
>  #else
>
>   

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