On Tue, Aug 31, 2021 at 02:35:00AM +0300, Max Gurtovoy wrote:
> No need to pre-allocate a big buffer for the IO SGL anymore. If a device
> has lots of deep queues, preallocation for the sg list can consume
> substantial amounts of memory. For HW virtio-blk device, nr_hw_queues
> can be 64 or 128 and each queue's depth might be 128. This means the
> resulting preallocation for the data SGLs is big.
>
> Switch to runtime allocation for SGL for lists longer than 2 entries.
> This is the approach used by NVMe drivers so it should be reasonable for
> virtio block as well. Runtime SGL allocation has always been the case
> for the legacy I/O path so this is nothing new.
>
> The preallocated small SGL depends on SG_CHAIN so if the ARCH doesn't
> support SG_CHAIN, use only runtime allocation for the SGL.
>
> Signed-off-by: Max Gurtovoy <mgurtovoy@xxxxxxxxxx>
> Reviewed-by: Israel Rukshin <israelr@xxxxxxxxxx>
> ---
> drivers/block/virtio_blk.c | 37 ++++++++++++++++++++++---------------
> 1 file changed, 22 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 77e8468e8593..9a4c5d428b58 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -24,6 +24,12 @@
> /* The maximum number of sg elements that fit into a virtqueue */
> #define VIRTIO_BLK_MAX_SG_ELEMS 32768
>
> +#ifdef CONFIG_ARCH_NO_SG_CHAIN
> +#define VIRTIO_BLK_INLINE_SG_CNT 0
> +#else
> +#define VIRTIO_BLK_INLINE_SG_CNT 2
> +#endif
> +
> static int virtblk_queue_count_set(const char *val,
> const struct kernel_param *kp)
> {
> @@ -99,7 +105,7 @@ struct virtio_blk {
> struct virtblk_req {
> struct virtio_blk_outhdr out_hdr;
> u8 status;
> - struct scatterlist sg[];
> + struct sg_table sg_table;
Please keep the sg flexible array member here instead of the pointer
arithmetics that is added instead below.
> + err = sg_alloc_table_chained(&vbr->sg_table,
> + blk_rq_nr_phys_segments(req),
> + vbr->sg_table.sgl,
> + VIRTIO_BLK_INLINE_SG_CNT);
> + if (err)
> + return BLK_STS_RESOURCE;
> +
This will BUG() for requests without segments (fush and discard). You
probably want a separate helper to actually map data in the, extending
the big switch on the op. While we're at it, the blk_mq_start_request
should also move as close as possible to the actual sending of the
request to the host.
You'll also need to select SG_POOL now that you're using these
functions.
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