From: Vasily Tarasov There is a small problem in handling page bounce. At the moment blk_max_pfn equals max_pfn, which is in fact not maximum possible _number_ of a page frame, but the _amount_ of page frames. For example for the 32bit x86 node with 4Gb RAM, max_pfn = 0x100000, but not 0xFFFF. request_queue structure has a member q->bounce_pfn and queue needs bounce pages for the pages _above_ this limit. This routine is handled by blk_queue_bounce(), where the following check is produced: if (q->bounce_pfn >= blk_max_pfn) return; Assume, that a driver has set q->bounce_pfn to 0xFFFF, but blk_max_pfn equals 0x10000. In such situation the check above fails and for each bio we always fall down for iterating over pages tied to the bio. I want to notice, that for quite a big range of device drivers (ide, md, ...) such problem doesn't happen because they use BLK_BOUNCE_ANY for bounce_pfn. BLK_BOUNCE_ANY is defined as blk_max_pfn << PAGE_SHIFT, and then the check above doesn't fail. But for other drivers, which obtain reuired value from drivers, it fails. For example sata_nv uses ATA_DMA_MASK or dev->dma_mask. I propose to use (max_pfn - 1) for blk_max_pfn. And the same for blk_max_low_pfn. The patch also cleanses some checks related with bounce_pfn. Signed-off-by: Vasily Tarasov Acked-by: Jens Axboe Signed-off-by: Andrew Morton --- block/ll_rw_blk.c | 6 +++--- mm/bounce.c | 2 +- 2 files changed, 4 insertions(+), 4 deletions(-) diff -puN block/ll_rw_blk.c~block-blk_max_pfn-is-somtimes-wrong block/ll_rw_blk.c --- a/block/ll_rw_blk.c~block-blk_max_pfn-is-somtimes-wrong +++ a/block/ll_rw_blk.c @@ -1213,7 +1213,7 @@ void blk_recount_segments(request_queue_ * considered part of another segment, since that might * change with the bounce page. */ - high = page_to_pfn(bv->bv_page) >= q->bounce_pfn; + high = page_to_pfn(bv->bv_page) > q->bounce_pfn; if (high || highprv) goto new_hw_segment; if (cluster) { @@ -3628,8 +3628,8 @@ int __init blk_dev_init(void) open_softirq(BLOCK_SOFTIRQ, blk_done_softirq, NULL); register_hotcpu_notifier(&blk_cpu_notifier); - blk_max_low_pfn = max_low_pfn; - blk_max_pfn = max_pfn; + blk_max_low_pfn = max_low_pfn - 1; + blk_max_pfn = max_pfn - 1; return 0; } diff -puN mm/bounce.c~block-blk_max_pfn-is-somtimes-wrong mm/bounce.c --- a/mm/bounce.c~block-blk_max_pfn-is-somtimes-wrong +++ a/mm/bounce.c @@ -204,7 +204,7 @@ static void __blk_queue_bounce(request_q /* * is destination page below bounce pfn? */ - if (page_to_pfn(page) < q->bounce_pfn) + if (page_to_pfn(page) <= q->bounce_pfn) continue; /* _