Re: [PATCH V2] x86: Fix an issue with invalid ACPI NUMA config

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On Tue, 29 Jan 2019 13:05:56 -0600
Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:

> On Tue, Jan 29, 2019 at 09:51:05AM +0000, Jonathan Cameron wrote:
> > On Mon, 28 Jan 2019 17:13:22 -0600
> > Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:  
> > > On Mon, Jan 28, 2019 at 11:31:08AM +0000, Jonathan Cameron wrote:  
> > > > On Thu, 20 Dec 2018 13:57:14 -0600
> > > > Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:    
> > > > > On Thu, Dec 20, 2018 at 09:13:12AM -0800, Dave Hansen wrote:    
> > > > > > On 12/20/18 7:12 AM, Bjorn Helgaas wrote:      
> 
> > > The current patch proposes setting "numa_off=1" in the x86 version of
> > > dummy_numa_init(), on the assumption (from the changelog) that:
> > > 
> > >   It is invalid under the ACPI spec to specify new NUMA nodes using
> > >   _PXM if they have no presence in SRAT.
> > > 
> > > Do you have a reference for this?  I looked and couldn't find a clear
> > > statement in the spec to that effect.  The _PXM description (ACPI
> > > v6.2, sec 6.1.14) says that two devices with the same _PXM value are
> > > in the same proximity domain, but it doesn't seem to require an SRAT.  
> > 
> > No comment (feel free to guess why). *sigh*  
> 
> Secret interpretations of the spec are out of bounds.  But I think
> it's a waste of time to argue about whether _PXM without SRAT is
> valid.  Systems like that exist, and I think it's possible to do
> something sensible with them.
> 
> > > Maybe it results in an issue when we call kmalloc_node() using this
> > > _PXM value that SRAT didn't tell us about?  If so, that's reminiscent
> > > of these earlier discussions about kmalloc_node() returning something
> > > useless if the requested node is not online:
> > > 
> > >   https://lkml.kernel.org/r/1527768879-88161-2-git-send-email-xiexiuqi@xxxxxxxxxx
> > >   https://lore.kernel.org/linux-arm-kernel/20180801173132.19739-1-punit.agrawal@xxxxxxx/
> > > 
> > > As far as I know, that was never really resolved.  The immediate
> > > problem of using passing an invalid node number to kmalloc_node() was
> > > avoided by using kmalloc() instead.  
> > 
> > Yes, that's definitely still a problem (or was last time I checked)
> >   
> > > > Dave's response was that we needed to fix the underlying issue of
> > > > trying to allocate from non existent NUMA nodes.    
> 
> > > Bottom line, I totally agree that it would be better to fix the
> > > underlying issue without trying to avoid it by disabling NUMA.  
> > 
> > I don't agree on this point.  I think two layers make sense.
> > 
> > If there is no NUMA description in DT or ACPI, why not just stop anything
> > from using it at all?  The firmware has basically declared there is no
> > point, why not save a bit of complexity (and use an existing tested code
> > path) but setting numa_off?  
> 
> Firmware with a _PXM does have a NUMA description.

Most of the meaning is lost.  It applies some grouping but no info
on the relative distance between that any anywhere else.
So perhaps 'some' description.

> 
> > However, if there is NUMA description, but with bugs then we should
> > protect in depth.  A simple example being that we declare 2 nodes, but
> > then use _PXM for a third. I've done that by accident and blows up
> > in a nasty fashion (not done it for a while, but probably still true).
> > 
> > Given DSDT is only parsed long after SRAT we can just check on _PXM
> > queries.  Or I suppose we could do a verification parse for all _PXM
> > entries and put out some warnings if they don't match SRAT entries?  
> 
> I'm assuming the crash happens when we call kmalloc_node() with a node
> not mentioned in SRAT.  I think that's just sub-optimal implementation
> in kmalloc_node().
> 
> We *could* fail the allocation and return a NULL pointer, but I think
> even that is excessive.  I think we should simply fall back to
> kmalloc().  We could print a one-time warning if that's useful.
> 
> If kmalloc_node() for an unknown node fell back to kmalloc(), would
> anything else be required?

It will deal with that case, but it may not be the only one.
I think there are interrupt related issues as well, but will have to check.

> 
> > > > Whilst I agree with that in principle (having managed to provide
> > > > tables doing exactly that during development a few times!), I'm not
> > > > sure the path to doing so is clear and so this has been stalled for
> > > > a few months.  There is to my mind still a strong argument, even
> > > > with such protection in place, that we should still be short cutting
> > > > it so that you get the same paths if you deliberately disable numa,
> > > > and if you have no SRAT and hence can't have NUMA.    
> > > 
> > > I guess we need to resolve the question of whether NUMA without SRAT
> > > is possible.  
> > 
> > It's certainly unclear of whether it has any meaning.  If we allow for
> > the fact that the intent of ACPI was never to allow this (and a bit
> > of history checking verified this as best as anyone can remember),
> > then what do we do with the few platforms that do use _PXM to nodes that
> > haven't been defined?  
> 
> We *could* ignore any _PXM that mentions a proximity domain not
> mentioned by an SRAT.  That seems a little heavy-handed because it
> means every possible proximity domain must be described up front in
> the SRAT, which limits the flexibility of hot-adding entire nodes
> (CPU/memory/IO).
> 
> But I think it's possible to make sense of a _PXM that adds a
> proximity domain not mentioned in an SRAT, e.g., if a new memory
> device and a new I/O device supply the same _PXM value, we can assume
> they're close together.  If a new I/O device has a previously unknown
> _PXM, we may not be able to allocate memory near it, but we should at
> least be able to allocate from a default zone.

I would like to know if this is real before we support it though.
We have a known platform that does it.  That platform might as well
not bother as I understand it as it doesn't have memory in those nodes.

I'll be honest though I'm happy with fixing it the hard way and
dropping the numa_off = 1 for arm if that is the consensus.

Jonathan

> 
> Bjorn





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