> I was thinking about a use case where userland would pin an address > without FOLL_WRITE, because the PTE for that address is not going to > be writable, but some device via DMA will write to it. Now, if we got > a zero page we have a problem... If this usecase is not valid then the > fix for movable zero page is make the zero page always come from a > non-movable zone so we do not need to isolate it during migration, and > so the memory can be offlined later. I looked into making zero_page non-movable, and I am confused here. huge zero page is already not movable: get_huge_zero_page() zero_page = alloc_pages((GFP_TRANSHUGE | __GFP_ZERO) & ~__GFP_MOVABLE, ... Base zero page can be in a movable zone, which is a bug: if there are references to zero page, that page cannot be migrated, and we won't be hot-remove memory area where that page is located. On x86, zero page should always come from the bottom 4G of physical memory / DMA32 ZONE. However, I see that sometimes it is not (I reproduce in QEMU emulator): QEMU instance with 16G of memory and kernelcore=5G Boot#1: zero_pfn 48a8d zero_pfn zone: ZONE_DMA32 Boot#2: zero_pfn 20168d zero_pfn zone: ZONE_MOVABLE (???) The problem is that the x86 zero page comes from the .bss segment: https://soleen.com/source/xref/linux/arch/x86/kernel/head_64.S?r=31d85460#583 Which, I thought would always be set within the first 4G of physical memory. What is going on here? Pasha
