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. > 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? > > > 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. Bjorn
