netdev - Re: [PATCH v2 01/21] scatterlist: Introduce sg

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Date:   Thu, 27 Apr 2017 09:44:18 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-raid@...r.kernel.org,
        linux-mmc@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-scsi@...r.kernel.org, open-iscsi@...glegroups.com,
        megaraidlinux.pdl@...adcom.com, sparmaintainer@...sys.com,
        devel@...verdev.osuosl.org, target-devel@...r.kernel.org,
        netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
        dm-devel@...hat.com,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        Jens Axboe <axboe@...nel.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [PATCH v2 01/21] scatterlist: Introduce sg_map helper functions



On 27/04/17 12:53 AM, Christoph Hellwig wrote:
> I think you'll need to follow the existing kmap semantics and never
> fail the iomem version either.  Otherwise you'll have a special case
> that's almost never used that has a different error path.
>
> Again, wrong way.  Suddenly making things fail for your special case
> that normally don't fail is a receipe for bugs.

I don't disagree but these restrictions make the problem impossible to
solve? If there is iomem behind a page in an SGL and someone tries to
map it, we either have to fail or we break iomem safety which was your
original concern.

Logan