Messages in this thread | | | Date | Tue, 5 Apr 2016 00:45:08 +0100 | From | Al Viro <> | Subject | Re: [RFC] weird semantics of SG_DXFER_TO_FROM_DEV in BLK_DEV_SKD (drivers/block/skd*) |
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On Mon, Apr 04, 2016 at 08:50:42PM +0100, Al Viro wrote:
> What happens if somebody issues SG_IO with 256-segment vector, each segment > 1 byte long and page-aligned? Will the driver really be happy with the > resulting request, as long as it hasn't claimed non-zero queue_virt_boundary? > Because AFAICS we'll get a request with a pile of bvecs, each with > ->bv_offset equal to 0 and ->bv_len equal to 1; can that really work?
OK, it really doesn't make sense. What happened, AFAICS, is that when blk_rq_map_user_iov() has grown a "misaligned, need to copy" code, the check had been mishandled - rather than checking both the base and the length of segments, as blk_rq_map_{user,kern} used to do (and as ..._kern is still doing) it checked only the base.
Then in "block: use blk_rq_map_user_iov to implement blk_rq_map_user" you've missed that problem, which got us the current situaiton. Note that e.g. PIO case of libata really wants copy in case of 500 bytes + 12 bytes vector - it'll splat the last 12 bytes adjacent to the end of the first segment, etc.
AFAICS, what we need there is simply nr_pages = iov_iter_npages(iter); alignment = iov_iter_alignment(iter); if (alignment & (queue_dma_alignment(q) | q->dma_pad_mask)) copy = true; and I really wonder if we care about special-casing the situation when the ends are not aligned to queue_virt_boundary(q). If we don't, we might as well add queue_virt_boundary(q) to the mask we are checking. If we do, it's not hard to add a variant that would calculate both the alignment and alignment for internal boundaries...
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