On Fri, Apr 19, 2019 at 08:56:24AM +0200, Christoph Hellwig wrote:
> While we generally allow scatterlists to have offsets larger than page
> size for an entry, and other subsystems like the crypto code make use of
> that, the block layer isn't quite ready for that. Flip the switch back
> to avoid them for now, and revisit that decision early in a merge window
> once the known offenders are fixed.
>
> Fixes: 8a96a0e40810 ("block: rewrite blk_bvec_map_sg to avoid a nth_page call")
> Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
(cautious)
Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx>
Cautious because there are currently a number if crashes and runtime
warnings in -next. I don't think any are related to the block code,
but I may be wrong.
Guenter
> ---
> block/blk-merge.c | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 247b17f2a0f6..21e87a714a73 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -474,9 +474,21 @@ static unsigned blk_bvec_map_sg(struct request_queue *q,
> while (nbytes > 0) {
> unsigned offset = bvec->bv_offset + total;
> unsigned len = min(get_max_segment_size(q, offset), nbytes);
> + struct page *page = bvec->bv_page;
> +
> + /*
> + * Unfortunately a fair number of drivers barf on scatterlists
> + * that have an offset larger than PAGE_SIZE, despite other
> + * subsystems dealing with that invariant just fine. For now
> + * stick to the legacy format where we never present those from
> + * the block layer, but the code below should be removed once
> + * these offenders (mostly MMC/SD drivers) are fixed.
> + */
> + page += (offset >> PAGE_SHIFT);
> + offset &= ~PAGE_MASK;
>
> *sg = blk_next_sg(sg, sglist);
> - sg_set_page(*sg, bvec->bv_page, len, offset);
> + sg_set_page(*sg, page, len, offset);
>
> total += len;
> nbytes -= len;
> --
> 2.20.1
>
