Re: swiotlb-xen: provide the "max_mapping_size" method

[Mike Snitzer <snitzer@xxxxxxxxxx>]

On Mon, Nov 06 2023 at 10:16P -0500,
Keith Busch <kbusch@xxxxxxxxxx> wrote:

> On Mon, Nov 06, 2023 at 03:59:40PM +0100, Mikulas Patocka wrote:
> > There's a bug that when using the XEN hypervisor with dm-crypt on NVMe, 
> > the kernel deadlocks [1].
> > 
> > The deadlocks are caused by inability to map a large bio vector -
> > dma_map_sgtable always returns an error, this gets propagated to the block
> > layer as BLK_STS_RESOURCE and the block layer retries the request
> > indefinitely.
> > 
> > XEN uses the swiotlb framework to map discontiguous pages into contiguous
> > runs that are submitted to the PCIe device. The swiotlb framework has a
> > limitation on the length of a mapping - this needs to be announced with
> > the max_mapping_size method to make sure that the hardware drivers do not
> > create larger mappings.
> > 
> > Without max_mapping_size, the NVMe block driver would create large
> > mappings that overrun the maximum mapping size.
> > 
> > [1] https://lore.kernel.org/stable/ZTNH0qtmint%2FzLJZ@mail-itl/
> 
> This should be a "Link:" tag.
> 
> > Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> > Reported-by: Marek Marczykowski-G'orecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > Tested-by: Marek Marczykowski-G'orecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > Suggested-by: Keith Busch <kbusch@xxxxxxxxxx>
> 
> I was about to send the same thing. I did a little more than suggest
> this: it's is the very patch I wrote for testing, minus the redundant
> nvme bits! But since you already have a commit message for it...
> 
> Acked-by: Keith Busch <kbusch@xxxxxxxxxx>

No, this patch should be attributed to you Keith.

Mikulas, I like that you ran with getting a fix prepared but please
update the patch so Keith is the author and use Link: as suggested for
the v2. Note: you'll still use your Signed-off-by since you had a role
in getting this patch together (but please move yours to the end of
the header).

Mike


> 
> > Suggested-by: Christoph Hellwig <hch@xxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > 
> > ---
> >  drivers/xen/swiotlb-xen.c |    1 +
> >  1 file changed, 1 insertion(+)
> > 
> > Index: linux-stable/drivers/xen/swiotlb-xen.c
> > ===================================================================
> > --- linux-stable.orig/drivers/xen/swiotlb-xen.c     2023-11-03 
> > 17:57:18.000000000 +0100
> > +++ linux-stable/drivers/xen/swiotlb-xen.c  2023-11-06 15:30:59.000000000 
> > +0100
> > @@ -405,4 +405,5 @@ const struct dma_map_ops xen_swiotlb_dma
> >     .get_sgtable = dma_common_get_sgtable,
> >     .alloc_pages = dma_common_alloc_pages,
> >     .free_pages = dma_common_free_pages,
> > +   .max_mapping_size = swiotlb_max_mapping_size,
> >  };
>