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

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Message-ID: <MWHPR12MB16002A374CBB41BB201BA061C87A0@MWHPR12MB1600.namprd12.prod.outlook.com>
Date:   Mon, 25 Sep 2017 20:19:21 +0000
From:   Casey Leedom <leedom@...lsio.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "Harsh Jain" <Harsh@...lsio.com>
CC:     "joro@...tes.org" <joro@...tes.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michael Werner <werner@...lsio.com>
Subject: Re: DMA error when sg->offset value is greater than PAGE_SIZE in
 Intel IOMMU

| From: David Woodhouse <dwmw2@...radead.org>
| Sent: Monday, September 25, 2017 11:45 AM
|
| On Wed, 2017-09-20 at 16:01 +0800, Herbert Xu wrote:
| > Harsh Jain <Harsh@...lsio.com> wrote:
| > >
| > > While debugging DMA mapping error in chelsio crypto driver we
| > observed that when scatter/gather list received by driver has some
| > entry with page->offset > 4096 (PAGE_SIZE). It starts giving DMA
| > error.  Without IOMMU it works fine.
| >
| > This is not a bug.  The network stack can and will feed us such
| > SG lists.
|
| Hm? Under what circumstances is the offset permitted to be >
| PAGE_SIZE?

  As I noted earlier, this is an area of the kernel with which I'm not super
familiar.  Both Herbert Xu and our local VM Expert have said that having
Scatter/Gather Lists with Offsets greater than Page Size is not a bug ...

  I'm mostly trying to help out keeping focus on this because Harsh is in
India (presumable enjoying a good night's sleep while we look at this.
Hopefully we'll have a present of a bug fix for him when he wakes up ... :-)

Casey