• Minchan Kim's avatar
    mm: fix MADV_[FREE|DONTNEED] TLB flush miss problem · 99baac21
    Minchan Kim authored
    Nadav reported parallel MADV_DONTNEED on same range has a stale TLB
    problem and Mel fixed it[1] and found same problem on MADV_FREE[2].
    
    Quote from Mel Gorman:
     "The race in question is CPU 0 running madv_free and updating some PTEs
      while CPU 1 is also running madv_free and looking at the same PTEs.
      CPU 1 may have writable TLB entries for a page but fail the pte_dirty
      check (because CPU 0 has updated it already) and potentially fail to
      flush.
    
      Hence, when madv_free on CPU 1 returns, there are still potentially
      writable TLB entries and the underlying PTE is still present so that a
      subsequent write does not necessarily propagate the dirty bit to the
      underlying PTE any more. Reclaim at some unknown time at the future
      may then see that the PTE is still clean and discard the page even
      though a write has happened in the meantime. I think this is possible
      but I could have missed some protection in madv_free that prevents it
      happening."
    
    This patch aims for solving both problems all at once and is ready for
    other problem with KSM, MADV_FREE and soft-dirty story[3].
    
    TLB batch API(tlb_[gather|finish]_mmu] uses [inc|dec]_tlb_flush_pending
    and mmu_tlb_flush_pending so that when tlb_finish_mmu is called, we can
    catch there are parallel threads going on.  In that case, forcefully,
    flush TLB to prevent for user to access memory via stale TLB entry
    although it fail to gather page table entry.
    
    I confirmed this patch works with [4] test program Nadav gave so this
    patch supersedes "mm: Always flush VMA ranges affected by zap_page_range
    v2" in current mmotm.
    
    NOTE:
    
    This patch modifies arch-specific TLB gathering interface(x86, ia64,
    s390, sh, um).  It seems most of architecture are straightforward but
    s390 need to be careful because tlb_flush_mmu works only if
    mm->context.flush_mm is set to non-zero which happens only a pte entry
    really is cleared by ptep_get_and_clear and friends.  However, this
    problem never changes the pte entries but need to flush to prevent
    memory access from stale tlb.
    
    [1] http://lkml.kernel.org/r/20170725101230.5v7gvnjmcnkzzql3@techsingularity.net
    [2] http://lkml.kernel.org/r/20170725100722.2dxnmgypmwnrfawp@suse.de
    [3] http://lkml.kernel.org/r/BD3A0EBE-ECF4-41D4-87FA-C755EA9AB6BD@gmail.com
    [4] https://patchwork.kernel.org/patch/9861621/
    
    [minchan@kernel.org: decrease tlb flush pending count in tlb_finish_mmu]
      Link: http://lkml.kernel.org/r/20170808080821.GA31730@bbox
    Link: http://lkml.kernel.org/r/20170802000818.4760-7-namit@vmware.com
    
    Signed-off-by: default avatarMinchan Kim <minchan@kernel.org>
    Signed-off-by: default avatarNadav Amit <namit@vmware.com>
    Reported-by: default avatarNadav Amit <namit@vmware.com>
    Reported-by: default avatarMel Gorman <mgorman@techsingularity.net>
    Acked-by: default avatarMel Gorman <mgorman@techsingularity.net>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Russell King <linux@armlinux.org.uk>
    Cc: Tony Luck <tony.luck@intel.com>
    Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
    Cc: "David S. Miller" <davem@davemloft.net>
    Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
    Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
    Cc: Jeff Dike <jdike@addtoit.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Nadav Amit <nadav.amit@gmail.com>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    99baac21
mm_types.h 18 KB