Hello everybody, We've discussed few times that is would be nice to allow huge pages to be mapped with 4k pages too. Here's my first attempt to actually implement this. It's early prototype and not stabilized yet, but I want to share it to discuss any potential show stoppers early. The main reason why we can't map THP with 4k is how refcounting on THP designed. It built around two requirements: - split of huge page should never fail; - we can't change interface of get_user_page(); To be able to split huge page at any point we have to track which tail page was pinned. It leads to tricky and expensive get_page() on tail pages and also occupy tail_page->_mapcount. Most split_huge_page*() users want PMD to be split into table of PTEs and don't care whether compound page is going to be split or not. The plan is: - allow split_huge_page() to fail if the page is pinned. It's trivial to split non-pinned page and it doesn't require tail page refcounting, so tail_page->_mapcount is free to be reused. - introduce new routine -- split_huge_pmd() -- to split PMD into table of PTEs. It splits only one PMD, not touching other PMDs the page is mapped with or underlying compound page. Unlike new split_huge_page(), split_huge_pmd() never fails. Fortunately, we have only few places where split_huge_page() is needed: swap out, memory failure, migration, KSM. And all of them can handle split_huge_page() fail. In new scheme we use tail_page->_mapcount is used to account how many time the tail page is mapped. head_page->_mapcount is used for both PMD mapping of whole huge page and PTE mapping of the firt 4k page of the compound page. It seems work fine, except the fact that we don't have a cheap way to check whether the page mapped with PMDs or not. Introducing split_huge_pmd() effectively allows THP to be mapped with 4k. It can break some kernel expectations. I.e. VMA now can start and end in middle of compound page. IIUC, it will break compactation and probably something else (any hints?). Also munmap() on part of huge page will not split and free unmapped part immediately. We need to be careful here to keep memory footprint under control. As side effect we don't need to mark PMD splitting since we have split_huge_pmd(). get_page()/put_page() on tail of THP is cheaper (and cleaner) now. I will continue with stabilizing this. The patchset also available on git[1]. Any commemnt? [1] git://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git thp/refcounting/v1 Kirill A. Shutemov (10): mm, thp: drop FOLL_SPLIT mm: change PageAnon() to work on tail pages thp: rename split_huge_page_pmd() to split_huge_pmd() thp: PMD splitting without splitting compound page mm, vmstats: new THP splitting event thp: implement new split_huge_page() mm, thp: remove infrastructure for handling splitting PMDs x86, thp: remove remove infrastructure for handling splitting PMDs futex, thp: remove special case for THP in get_futex_key thp: update documentation Documentation/vm/transhuge.txt | 95 ++++---- arch/mips/mm/gup.c | 4 - arch/powerpc/mm/hugetlbpage.c | 12 - arch/powerpc/mm/subpage-prot.c | 2 +- arch/s390/mm/gup.c | 13 +- arch/s390/mm/pgtable.c | 17 +- arch/sparc/mm/gup.c | 14 +- arch/x86/include/asm/pgtable.h | 9 - arch/x86/include/asm/pgtable_types.h | 2 - arch/x86/kernel/vm86_32.c | 6 +- arch/x86/mm/gup.c | 17 +- arch/x86/mm/pgtable.c | 14 -- fs/proc/task_mmu.c | 9 +- include/asm-generic/pgtable.h | 5 - include/linux/huge_mm.h | 48 +--- include/linux/hugetlb_inline.h | 9 +- include/linux/mm.h | 66 +----- include/linux/vm_event_item.h | 4 +- kernel/futex.c | 62 ++---- mm/gup.c | 18 +- mm/huge_memory.c | 412 ++++++++++------------------------- mm/internal.h | 31 +-- mm/memcontrol.c | 16 +- mm/memory.c | 20 +- mm/migrate.c | 7 +- mm/mprotect.c | 2 +- mm/mremap.c | 2 +- mm/pagewalk.c | 2 +- mm/pgtable-generic.c | 14 -- mm/rmap.c | 4 +- mm/swap.c | 285 +++++++----------------- mm/vmstat.c | 4 +- 32 files changed, 328 insertions(+), 897 deletions(-) -- 2.0.0.rc4 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>