lkml.org 
[lkml]   [2018]   [Mar]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH 3/4] nvmet-fc: Don't use the count returned by the dma_map_sg call
    From
    Date
    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

    \
     
     \ /
      Last update: 2018-03-29 18:25    [W:3.145 / U:0.160 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site