Messages in this thread | | | From | Linus Torvalds <> | Date | Thu, 18 Nov 2021 10:33:39 -0800 | Subject | Re: [BUG] WARNING: CPU: 3 PID: 1 at mm/debug_vm_pgtable.c:493 |
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On Thu, Nov 18, 2021 at 8:47 AM Steven Rostedt <rostedt@goodmis.org> wrote: > > Triggered it again with the new update: > > [ 24.751779] IPI shorthand broadcast: enabled > [ 24.761177] sched_clock: Marking stable (23431856262, 1329270511)->(28163092341, -3401965568) > [ 24.770495] device: 'cpu_dma_latency': device_add > [ 24.775232] PM: Adding info for No Bus:cpu_dma_latency > [ 24.780929] debug_vm_pgtable: [debug_vm_pgtable ]: Validating architecture page table helpers > [ 24.799490] mtrr_type_lookup() returned 0 (0)
Ok, so that's MTRR_TYPE_UNCACHABLE, and "uniform" is 0.
Anyway, either the mtrr code is confused, or more likely it just does the right thing, and pud_set_huge() is simply expected to return 0 in this situation, and that WARN_ON() in pud_huge_tests() is simply wrong to trigger at all.
I didn't look at what all the code in debug_vm_pgtable() is trying to set up to test. Honestly, it's all very opaque.
But I do notice that the pfn that the test uses ends up basically being something random, where the "fixed" pfn is
phys = __pa_symbol(&start_kernel); ... args->fixed_pud_pfn = __phys_to_pfn(phys & PUD_MASK);
rather than being an allocated real PUD-sized page. That can be a problem in itself.
So I think the problem is that depending on where the kernel is allocated, the fixed_pud_pfn ends up being in an area with MTRR settings. In fact, I'm surprised it's not *always* in that area, since presumabl;y you have the normal fixed MTRR issues with the 640k-1M range.
But I didn't look - probably the MTRR code doesn't actually check the special fixed MTRR's.
Anyway, I think that the end result is simply that the tests in mm/debug_vm_pgtable.c are simply buggy, and the WARN_ON() is not a sign of anything wrong in the mm, but with the tests themselves.
So the fixed_pud_pfn is dodgy, but it looks like the non-fixed 'pud_pfn' allocation may be dodgy too:
#ifdef CONFIG_CONTIG_ALLOC if (order >= MAX_ORDER) { page = alloc_contig_pages((1 << order), GFP_KERNEL, first_online_node, NULL);
because afaik, alloc_contig_pages() does allocate a contiguous region, but it doesn't necessarily allocate a _aligned_ contiguous region.
So I think _all_ those PUD tests are likely broken, but honestly, I don't know the code well enough to be entirely sure, I'm just seeing code that looks dodgy to me.
I don't think the breakage is x86-specific. Quite the reverse. I think the x86 code just happens to randomly show it when some MTRR ends up being used.
Maybe pfn_pud() should verify that it's actually given an aligned argument?
Gavin, Anshuman? Feel free to tell me what I missed.
Otherwise, we should disable those PUD tests (or fix them, of course).
For now, I consider this WARN_ON() to be a failure of the testing infrastructure, not of the VM code.
Linus
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