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
