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When the loop completes, pfn is at or beyond end_pfn. Passing it to can_unmap_without_split() therefore checks a range after the one being offlined. Consequently, a valid request can be rejected based on the following range, while a request that would split a leaf mapping can be accepted if the shifted range can be unmapped without a split. This was observed with CXL DAX memory, where the final memory block was incorrectly allowed to be offlined. Pass arg->start_pfn into can_unmap_without_split() so it checks the requested range. Fixes: 95a58852b0e5 ("arm64/mm: Reject memory removal that splits a kernel leaf mapping") Signed-off-by: Richard Cheng Reviewed-by: Anshuman Khandual --- Changelog: V1->v2: - Fix the typo in the subject - Clarify how pfn advances and which range was incorrectly checked The bug occurred on a machine with CXL Type-3 device. Branch: cxl-next Before 95a58852b0e5: """ $ sudo echo offline > memory557056/state write error: Operation not permitted $ sudo dmesg ---[snip]--- [440008000000 440010000000] splits a leaf entry in linear map $ sudo echo offline > memory558079/state $ cat memory558079/state offline """ After: """ $ sudo echo offline > memory557056/state write error: Operation not permitted [440000000000 440008000000] splits a leaf entry in linear map $ sudo echo offline > memory558079/state write error: Operation not permitted [441ff8000000 442000000000] splits a leaf entry in linear map """ My fix corrects the checked range and prevents the false acceptance. Best regards, Richard Cheng. --- arch/arm64/mm/mmu.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index a25d8beacc83..18a8b0d3714e 100644 --- a/arch/arm64/mm/mmu.c +++ b/arch/arm64/mm/mmu.c @@ -2194,7 +2194,7 @@ static int prevent_memory_remove_notifier(struct notifier_block *nb, } } - if (!can_unmap_without_split(pfn, arg->nr_pages)) + if (!can_unmap_without_split(arg->start_pfn, arg->nr_pages)) return NOTIFY_BAD; return NOTIFY_OK; base-commit: 5ca04f3ba91f1773bbd5da6d9c654ccc1ba7831d -- 2.43.0