linux-kernel - Re: DMA error when sg->offset value is greater than PAGE

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Message-ID: <CAPcyv4hL92Ru5x-b_5s80EwO=w37M=KEr289zrCRSM_8juvDzw@mail.gmail.com>
Date:   Wed, 27 Sep 2017 10:13:51 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Casey Leedom <leedom@...lsio.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Harsh Jain <Harsh@...lsio.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        David Woodhouse <David.Woodhouse@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        Michael Werner <werner@...lsio.com>
Subject: Re: DMA error when sg->offset value is greater than PAGE_SIZE in
 Intel IOMMU

On Wed, Sep 27, 2017 at 9:31 AM, Casey Leedom <leedom@...lsio.com> wrote:
> | From: Dan Williams <dan.j.williams@...el.com>
> | Sent: Tuesday, September 26, 2017 9:10 AM
> |
> | On Tue, Sep 26, 2017 at 9:06 AM, Casey Leedom <leedom@...lsio.com> wrote:
> | > | From: Robin Murphy <robin.murphy@....com>
> | > | Sent: Tuesday, September 26, 2017 7:22 AM
> | > |...
> | > ...
> | >   Regardless, it seems that you agree that there's an issue with the Intel
> | > I/O MMU support code with regard to the legal values which a (struct
> | > scatterlist) can take on?  I still can't find any documentation for this
> | > and, personally, I'm a bit baffled by a Page-oriented Scatter/Gather List
> | > representation where [Offset, Offset+Length) can reside outside the Page.
> |
> | Consider the case where the page represents a huge page, then an
> | offset greater than PAGE_SIZE (up to HPAGE_SIZE) makes sense.
>
>   Okay, but whatever the underlaying Page Size is, should [Offset,
> Offset+Length) completely reside within the referenced Page?  I'm just
> trying to understand the Invariance Conditions which are assumed by all of
> the code which processes Scatter/gather Lists ...

As far as I can see "Offset can be greater than PAGE_SIZE" is the only
safe assumption for core code.