linux-kernel - Re: [PATCH 4/4] nvmet-fc: Use new SGL alloc/free helper for requests

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Message-ID: <20180329181545.GA16116@lst.de>
Date:   Thu, 29 Mar 2018 20:15:45 +0200
From:   Christoph Hellwig <hch@....de>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     James Smart <james.smart@...adcom.com>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>
Subject: Re: [PATCH 4/4] nvmet-fc: Use new SGL alloc/free helper for
        requests

On Thu, Mar 29, 2018 at 11:02:10AM -0600, Logan Gunthorpe wrote:
> Per the bug in the previous patch, I don't think that was ever a valid
> assumption. It doesn't have anything to do with the sgl_alloc change
> either. The dma_map interface is allowed to merge SGLs and that's why it
> can return fewer nents than it was passed. I'm not sure how many or
> which DMA ops actually do this which is why it hasn't actually
> manifested itself as a bug; but it is part of how the interface is
> specified to work.

Most iommus can, it just is very unlikely do happen in block drivers
given that the block layer already merges segments higher up.