Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory
From: David Hildenbrand <hidden>
Date: 2021-03-04 08:14:53
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On 04.03.21 04:31, Anshuman Khandual wrote:
On 3/4/21 2:54 AM, Will Deacon wrote:quoted
On Wed, Mar 03, 2021 at 07:04:33PM +0000, Catalin Marinas wrote:quoted
On Thu, Feb 11, 2021 at 01:35:56PM +0100, David Hildenbrand wrote:quoted
On 11.02.21 13:10, Anshuman Khandual wrote:quoted
On 2/11/21 5:23 PM, Will Deacon wrote:quoted
... 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.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. -- Thanks, David / dhildenb _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel