Re: [PATCH v5 4/5] sd: limit to use write life hints

[Christoph Hellwig <hch@lst.de>]

> > If the device (or file system, which really needs to be in control
> > for actual files vs just block devices) does not support all 256
> > we need to reduce them to less than that.  The kernel can help with
> > that a bit if the streams have meanings (collapsing temperature levels
> > that are close), but not at all if they don't have meanings. 
> 
> Current patch (nvme) does what you mentioned above.
> Pasting the fragment that maps potentially large placement-hints to the 
> last valid placement-id.
> 
> +static inline void nvme_assign_placement_id(struct nvme_ns *ns,
> +					struct request *req,
> +					struct nvme_command *cmd)
> +{
> +	u8 h = umin(ns->head->nr_plids - 1,
> +				WRITE_PLACEMENT_HINT(req->write_hint));
> +
> +	cmd->rw.control |= cpu_to_le16(NVME_RW_DTYPE_DPLCMT);
> +	cmd->rw.dsmgmt |= cpu_to_le32(ns->head->plids[h] << 16);
> +}
> 
> But this was just an implementation choice (and not a failure avoidance 
> fallback).

And it completely fucks thing up as I said.  If I have an application
that wants to separate streams I need to know how many stream I
have available, and not fold all higher numbers into the last one
available.