[RFC PATCH] nvme-pci: Bounce buffer for interleaved metadata

[Sagi Grimberg]

Hi Keith,

> NVMe namespace formats allow the possibility for metadata as extended
> LBAs. These require the memory interleave block and metadata in a single
> virtually contiguous buffer.
> 
> The Linux block layer, however, maintains metadata and data in separate
> buffers, which is unusable for NVMe drives using interleaved metadata
> formats.

That's not specific for NVMe, I vaguely recall we had this discussion
for passthru scsi devices (in scsi target context) 5 years ago...
It makes sense for FC (and few RDMA devices) that already get
interleaved metadata from the wire to keep it as is instead of
scattering it if the backend nvme device supports interleaved mode...

I would say that this support for this is something that belongs in the
block layer. IIRC mkp also expressed interest in using preadv2/pwritev2
to for user-space to use DIF with some accounting on the iovec so maybe
we can add a flag for interleaved metadata.

> This patch will enable such formats by allocating a bounce buffer
> interleaving the block and metadata, copying the everythign into the
> buffer for writes, or from it for reads.
> 
> I dislike this feature intensely. It is incredibly slow and enough memory
> overhead to make this not very useful for reclaim, but it's possible
> people will leave me alone if the Linux nvme driver accomodated this
> format.

Not only that it will be non-useful, but probably unusable. Once upon of
time iser did bounce buffering with large contiguous atomic allocations,
it just doesn't work... especially with nvme large number of deep queues
that can host commands of MDTS bytes each.

If we end up keeping it private to nvme, the first comment I'd give you
is to avoid high-order allocations, you'll see lots of bug reports
otherwise...