linux-kernel - Re: dma api errors with swiotlb

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Message-ID: <20190904125538.GA30177@infradead.org>
Date:   Wed, 4 Sep 2019 05:55:38 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: dma api errors with swiotlb

On Wed, Sep 04, 2019 at 02:54:26PM +0200, Thomas Hellström (VMware) wrote:
> On 9/4/19 2:17 PM, Christoph Hellwig wrote:
> > A call to dma_max_mapping_size() to limit the maximum I/O size solves
> > that problem.  With the latest kernel that should actually be done
> > automatically by the SCSI midlayer for you.
> 
> Hmm, OK. I guess with a sufficient queue depth and many mappings waiting for
> DMA completion, the SWIOTLB may fill up anyway...
> 
> I'll see if I can come up with something.

You are supposed to return SCSI_MLQUEUE_HOST_BUSY in that case,
which means that the kernel won't send more commands until another
one completed.