Messages in this thread | | | Subject | Re: [PATCH 3/4] nvmet-fc: Don't use the count returned by the dma_map_sg call | From | James Smart <> | Date | Thu, 29 Mar 2018 09:24:36 -0700 |
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On 3/29/2018 9:07 AM, Logan Gunthorpe wrote: > When allocating an SGL, the fibre channel target uses the number > of entities mapped as the number of entities in a given scatter > gather list. This is incorrect. > > The DMA-API-HOWTO document gives this note: > > The 'nents' argument to the dma_unmap_sg call must be > the _same_ one you passed into the dma_map_sg call, > it should _NOT_ be the 'count' value _returned_ from the > dma_map_sg call. > > The fc code only stores the count value returned form the dma_map_sg() > call and uses that value in the call to dma_unmap_sg(). > > The dma_map_sg() call will return a lower count than nents when multiple > SG entries were merged into one. This implies that there will be fewer > DMA address and length entries but the original number of page entries > in the SGL. So if this occurs, when the SGL reaches nvmet_execute_rw(), > a bio would be created with fewer than the total number of entries. > > As odd as it sounds, and as far as I can tell, the number of SG entries > mapped does not appear to be used anywhere in the fc driver and therefore > there's no current need to store it. > > Signed-off-by: Logan Gunthorpe <logang@deltatee.com> > Cc: James Smart <james.smart@broadcom.com> > Cc: Christoph Hellwig <hch@lst.de> > Cc: Sagi Grimberg <sagi@grimberg.me> > Fixes: c53432030d8642 ("nvme-fabrics: Add target support for FC transport") > ---
Signed-off-by: James Smart <james.smart@broadcom.com>
Patch looks fine.
As for "not used anywhere", be careful as the structure being prepped is passed from the nvme-fc transport to an underlying lldd. So the references would likely be in the lldd.
-- james
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