Messages in this thread | | | Subject | Re: [BUG] WARNING: CPU: 3 PID: 1 at mm/debug_vm_pgtable.c:493 | From | Anshuman Khandual <> | Date | Mon, 22 Nov 2021 12:01:47 +0530 |
| |
On 11/19/21 12:03 AM, Linus Torvalds wrote: > 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.
Hi Linus,
These PUD tests have been subtle (including their problems as seen here in this report) on certain platforms. I will definitely take a detailed look, but probably after an week (leave, travel etc). Thank you.
- Anshuman
| |