On 3/4/21 3:06 PM, Will Deacon wrote: > On Thu, Mar 04, 2021 at 09:12:31AM +0100, David Hildenbrand wrote: >> On 04.03.21 04:31, Anshuman Khandual wrote: >>> On 3/4/21 2:54 AM, Will Deacon wrote: >>>> On Wed, Mar 03, 2021 at 07:04:33PM +0000, Catalin Marinas wrote: >>>>> On Thu, Feb 11, 2021 at 01:35:56PM +0100, David Hildenbrand wrote: >>>>>> On 11.02.21 13:10, Anshuman Khandual wrote: >>>>>>> On 2/11/21 5:23 PM, Will Deacon wrote: >>>>>>>> ... and dropped. These patches appear to be responsible for a boot >>>>>>>> regression reported by CKI: >>>>>>> >>>>>>> Ahh, boot regression ? These patches only change the behaviour >>>>>>> for non boot memory only. >>>>>>> >>>>>>>> https://lore.kernel.org/r/cki.8D1CB60FEC.K6NJMEFQPV@xxxxxxxxxx >>>>>>> >>>>>>> Will look into the logs and see if there is something pointing to >>>>>>> the problem. >>>>>> >>>>>> It's strange. One thing I can imagine is a mis-detection of early sections. >>>>>> However, I don't see that happening: >>>>>> >>>>>> In sparse_init_nid(), we: >>>>>> 1. Initialize the memmap >>>>>> 2. Set SECTION_IS_EARLY | SECTION_HAS_MEM_MAP via >>>>>> sparse_init_one_section() >>>>>> >>>>>> Only hotplugged sections (DIMMs, dax/kmem) set SECTION_HAS_MEM_MAP without >>>>>> SECTION_IS_EARLY - which is correct, because these are not early. >>>>>> >>>>>> So once we know that we have valid_section() -- SECTION_HAS_MEM_MAP is set >>>>>> -- early_section() should be correct. >>>>>> >>>>>> Even if someone would be doing a pfn_valid() after >>>>>> memblocks_present()->memory_present() but before >>>>>> sparse_init_nid(), we should be fine (!valid_section() -> return 0). >>>>> >>>>> I couldn't figure out how this could fail with Anshuman's patches. >>>>> Will's suspicion is that some invalid/null pointer gets dereferenced >>>>> before being initialised but the only case I see is somewhere in >>>>> pfn_section_valid() (ms->usage) if valid_section() && !early_section(). >>>>> >>>>> Assuming that we do get a valid_section(ms) && !early_section(ms), is >>>>> there a case where ms->usage is not initialised? I guess races with >>>>> section_deactivate() are not possible this early. >>>>> >>>>> Another situation could be that pfn_valid() returns true when no memory >>>>> is mapped for that pfn. >>>> >>>> The case I wondered about was __pfn_to_section() with a bogus pfn, since >>>> with patch 2/2 we call that *before* checking that pfn_to_section_nr() is >>>> sane. >>> >>> Right, that is problematic. __pfn_to_section() should not be called without >>> first validating pfn_to_section_nr(), as it could cause out-of-bound access >>> on mem_section buffer. Will fix that order but as there is no test scenario >>> which is definitive for this reported regression, how should we ensure that >>> it fixes the problem ? >> >> Oh, right, I missed that in patch #2. (and when comparing to generic >> pfn_valid()). >> >> I thought bisecting pointed at patch #1, that's why I didn't even have >> another look at patch #2. Makes sense. > > I don't think we ever bisected it beyond these two patches, so it could > be either of them. Anshuman -- please work with Veronika on this, as she > has access to the problematic machine and was really helpful in debugging > this last time. Sure, will respin the patch series with a fix for [PATCH 2/2] as discussed and then follow up with Veronika to recreate the problem.
