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

Pavel Begunkov
2020-12-23T20:37:09Z

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?

--
Pavel Begunkov