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. > As it happens early during boot, I doubt that some NVDIMMs that get detected > and added early during boot as system RAM (via dax/kmem) are the problem. It is indeed very early, we can't even get the early console output. Debugging this is even harder as it's only misbehaving on a board we don't have access to. On the logic in this patch, is the hot-added memory always covering a full subsection? The arm64 pfn_valid() currently relies on memblock_is_map_memory() but the patch changes it to pfn_section_valid(). So if hot-added memory doesn't cover the full subsection, it may return true even if the pfn is not mapped. Regarding the robustness of the pfn_valid for ZONE_DEVICE memory, could we instead have a SECTION_IS_DEVICE flag and only check for that as not to disturb the hotplugged memory check via memblock_is_map_memory()? -- Catalin
