From: Andrea Arcangeli Fix a post-2.624 regression introduced by commit 419c434c35614609fd0c79d335c134bf4b88b30b Author: Yang Shi Date: Tue Mar 4 11:20:51 2008 +0100 Fix DMA access of block device in 64-bit kernel on some non-x86 systems with 4GB or upper 4GB memory For some non-x86 systems with 4GB or upper 4GB memory, we need increase the range of addresses that can be used for direct DMA in 64-bit kernel. This is crashing at boot my lowmem reserved RAM patch. This is causing GFP_DMA allocations at boot for no good reason. It crashes in my case because there's no ram below 16M available to linux. 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. 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 Tested-by: Yang Shi Cc: Jens Axboe Cc: "Rafael J. Wysocki" Signed-off-by: Andrew Morton --- block/blk-settings.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff -puN block/blk-settings.c~block-fix-the-setting-of-the-bounce-limit block/blk-settings.c --- a/block/blk-settings.c~block-fix-the-setting-of-the-bounce-limit +++ a/block/blk-settings.c @@ -140,7 +140,7 @@ void blk_queue_bounce_limit(struct reque /* 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 _