You are using the term "Sub-section memory hotplug support", but is it actually what you mean? To rephrase, aren't we talking here about "Sub-section device memory hotplug support" or similar?
Specifically it is support for passing @start and @size arguments to arch_add_memory() that are not section aligned. It's not limited to "device memory" which is otherwise not a concept that arch_add_memory() understands, it just groks spans of pfns.
Okay, so everything that does not have a memory block devices as of now.
Reason I am asking is because I wonder how that would interact with the memory block device infrastructure and hotplugging of system ram - add_memory()/add_memory_resource(). I *assume* you are not changing the add_memory() interface, so that one still only works with whole sections (or well, memory_block_size_bytes()) - check_hotplug_memory_range().
Like you found below, the implementation enforces that add_memory_*() interfaces maintain section alignment for @start and @size.
In general, mix and matching system RAM and persistent memory per section, I am not a friend of that.
You have no choice. The platform may decide to map PMEM and System RAM in the same section because the Linux section is too large compared to typical memory controller mapping granularity capability.
I might be very wrong here, but do we actually care about something like 64MB getting lost in the cracks? I mean if it simplifies core MM, let go of the couple of MB of system ram and handle the PMEM part only. Treat the system ram parts like memory holes we already have in ordinary sections (well, there we simply set the relevant struct pages to PG_reserved). Of course, if we have hundreds of unaligned devices and stuff will start to add up ... but I assume this is not the case?
Especially when it comes to memory block devices. But I am getting the feeling that we are rather targeting PMEM vs. PMEM with this patch series.
The collisions are between System RAM, PMEM regions, and PMEM namespaces (sub-divisions of regions that each need their own mapping lifetime).
Understood. I wonder if that PMEM only mapping (including separate lifetime) could be handled differently. But I am absolutely no expert, just curious.
Quote patch7:
"The libnvdimm sub-system has suffered a series of hacks and broken workarounds for the memory-hotplug implementation's awkward section-aligned (128MB) granularity. For example the following backtrace is emitted when attempting arch_add_memory() with physical address ranges that intersect 'System RAM' (RAM) with 'Persistent Memory' (PMEM) within a given section:
WARNING: CPU: 0 PID: 558 at kernel/memremap.c:300 devm_memremap_pages+0x3b5/0x4c0 devm_memremap_pages attempted on mixed region [mem 0x200000000-0x2fbffffff flags 0x200] [..] Call Trace: dump_stack+0x86/0xc3 __warn+0xcb/0xf0 warn_slowpath_fmt+0x5f/0x80 devm_memremap_pages+0x3b5/0x4c0 __wrap_devm_memremap_pages+0x58/0x70 [nfit_test_iomap] pmem_attach_disk+0x19a/0x440 [nd_pmem]
Recently it was discovered that the problem goes beyond RAM vs PMEM collisions as some platform produce PMEM vs PMEM collisions within a
As side-noted by Michal, I wonder if PMEM vs. PMEM cannot rather be implemented "on top" of what we have right now. Or is this what we already have that you call "hacks in nvdimm" code? (no NVDIMM expert, sorry for the stupid questions)
It doesn't work, because even if the padding was implemented 100% correct, which thus far has failed to be the case, the platform may change physical alignments from one boot to the next for a variety of reasons.
Would ignoring the System RAM parts (as mentioned above) help or doesn't it make any difference in terms of complexity?
given section. The libnvdimm workaround for that case revealed that the libnvdimm section-alignment-padding implementation has been broken for a long while. A fix for that long-standing breakage introduces as many problems as it solves as it would require a backward-incompatible change to the namespace metadata interpretation. Instead of that dubious route [2], address the root problem in the memory-hotplug implementation."
The approach is taken is to observe that each section already maintains an array of 'unsigned long' values to hold the pageblock_flags. A single additional 'unsigned long' is added to house a 'sub-section active' bitmask. Each bit tracks the mapped state of one sub-section's worth of capacity which is SECTION_SIZE / BITS_PER_LONG, or 2MB on x86-64.
The implication of allowing sections to be piecemeal mapped/unmapped is that the valid_section() helper is no longer authoritative to determine if a section is fully mapped. Instead pfn_valid() is updated to consult the section-active bitmask. Given that typical memory hotplug still has deep "section" dependencies the sub-section capability is limited to 'want_memblock=false' invocations of arch_add_memory(), effectively only devm_memremap_pages() users for now.
Ah, there it is. And my point would be, please don't ever unlock something like that for want_memblock=true. Especially not for memory added after boot via device drivers (add_memory()).
I don't see a strong reason why not, as long as it does not regress existing use cases. It might need to be an opt-in for new tooling that is aware of finer granularity hotplug. That said, I have no pressing need to go there and just care about the arch_add_memory() capability for now.
Especially onlining/offlining of memory might end up very ugly. And that goes hand in hand with memory block devices. They are either online or offline, not something in between. (I went that path and Michal correctly told me why it is not a good idea)
I was recently trying to teach memory block devices who their owner is / of which type they are. Right now I am looking into the option of using drivers. Memory block devices that could belong to different drivers at a time are well ... totally broken. I assume it would still be a special case, though, but conceptually speaking about the interface it would be allowed.
Memory block devices (and therefore 1..X sections) should have one owner only. Anything else just does not fit.