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 ?
