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    SubjectRe: regression breaks lowmem reserved RAM
    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@qumranet.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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