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

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Date:   Thu, 27 Apr 2017 08:53:38 +0200
From:   Christoph Hellwig <hch@....de>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Christoph Hellwig <hch@....de>, 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 Wed, Apr 26, 2017 at 12:11:33PM -0600, Logan Gunthorpe wrote:
> Ok, well for starters I think you are mistaken about kmap being able to
> fail. I'm having a hard time finding many users of that function that
> bother to check for an error when calling it.

A quick audit of the arch code shows you're right - kmap can't fail
anywhere anymore.

> The main difficulty we
> have now is that neither of those functions are expected to fail and we
> need them to be able to in cases where the page doesn't map to system
> RAM. This patch series is trying to address it for users of scatterlist.
> I'm certainly open to other suggestions.

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.

> There are a fair number of cases in the kernel that do something like:
> 
> if (something)
>     x = kmap(page);
> else
>     x = kmap_atomic(page);
> ...
> if (something)
>     kunmap(page)
> else
>     kunmap_atomic(x)
> 
> Which just seems cumbersome to me.

Passing a different flag based on something isn't really much better.

> In any case, if you can accept an sg_kmap and sg_kmap_atomic api just
> say so and I'll make the change. But I'll still need a flags variable
> for SG_MAP_MUST_NOT_FAIL to support legacy cases that have no fail path
> and both of those functions will need to be pretty nearly replicas of
> each other.

Again, wrong way.  Suddenly making things fail for your special case
that normally don't fail is a receipe for bugs.