Re: [PATCH v1 0/6] no-copy bvec

Christoph Hellwig
2020-12-24T06:42:25Z

On Wed, Dec 23, 2020 at 08:32:45PM +0000, Pavel Begunkov wrote:
> On 23/12/2020 20:23, Douglas Gilbert wrote:
> > On 2020-12-23 11:04 a.m., James Bottomley wrote:
> >> On Wed, 2020-12-23 at 15:51 +0000, Christoph Hellwig wrote:
> >>> On Wed, Dec 23, 2020 at 12:52:59PM +0000, Pavel Begunkov wrote:
> >>>> Can scatterlist have 0-len entries? Those are directly translated
> >>>> into bvecs, e.g. in nvme/target/io-cmd-file.c and
> >>>> target/target_core_file.c. I've audited most of others by this
> >>>> moment, they're fine.
> >>>
> >>> For block layer SGLs we should never see them, and for nvme neither.
> >>> I think the same is true for the SCSI target code, but please double
> >>> check.
> >>
> >> Right, no-one ever wants to see a 0-len scatter list entry.?? The reason
> >> is that every driver uses the sgl to program the device DMA engine in
> >> the way NVME does.?? a 0 length sgl would be a dangerous corner case:
> >> some DMA engines would ignore it and others would go haywire, so if we
> >> ever let a 0 length list down into the driver, they'd have to
> >> understand the corner case behaviour of their DMA engine and filter it
> >> accordingly, which is why we disallow them in the upper levels, since
> >> they're effective nops anyway.
> >
> > When using scatter gather lists at the far end (i.e. on the storage device)
> > the T10 examples (WRITE SCATTERED and POPULATE TOKEN in SBC-4) explicitly
> > allow the "number of logical blocks" in their sgl_s to be zero and state
> > that it is _not_ to be considered an error.
>
> It's fine for my case unless it leaks them out of device driver to the
> net/block layer/etc. Is it?

None of the SCSI Command mentions above are supported by Linux,
nevermind mapped to struct scatterlist.