Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory | Date | Thu, 11 Feb 2021 13:35:56 +0100 |
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On 11.02.21 13:10, Anshuman Khandual wrote: > > > On 2/11/21 5:23 PM, Will Deacon wrote: >> On Fri, Feb 05, 2021 at 06:55:53PM +0000, Will Deacon wrote: >>> On Wed, Feb 03, 2021 at 09:20:39AM +0530, Anshuman Khandual wrote: >>>> On 2/2/21 6:26 PM, David Hildenbrand wrote: >>>>> On 02.02.21 13:51, Will Deacon wrote: >>>>>> On Tue, Feb 02, 2021 at 01:39:29PM +0100, David Hildenbrand wrote: >>>>>>> As I expressed already, long term we should really get rid of the arm64 >>>>>>> variant and rather special-case the generic one. Then we won't go out of >>>>>>> sync - just as it happened with ZONE_DEVICE handling here. >>>>>> >>>>>> Why does this have to be long term? This ZONE_DEVICE stuff could be the >>>>>> carrot on the stick :) >>>>> >>>>> Yes, I suggested to do it now, but Anshuman convinced me that doing a >>>>> simple fix upfront might be cleaner --- for example when it comes to >>>>> backporting :) >>>> >>>> Right. The current pfn_valid() breaks for ZONE_DEVICE memory and this fixes >>>> the problem in the present context which can be easily backported if required. >>>> >>>> Changing or rather overhauling the generic code with new configs as proposed >>>> earlier (which I am planning to work on subsequently) would definitely be an >>>> improvement for the current pfn_valid() situation in terms of maintainability >>>> but then it should not stop us from fixing the problem now. >>> >>> Alright, I've mulled this over a bit. I don't agree that this patch helps >>> with maintainability (quite the opposite, in fact), but perfection is the >>> enemy of the good so I'll queue the series for 5.12. However, I'll revert >>> the changes at the first sign of a problem, so please do work towards a >>> generic solution which can replace this in the medium term. >> >> ... 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@redhat.com > > 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).
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.
-- Thanks,
David / dhildenb
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