• Jérôme Glisse's avatar
    mm/mmu_notifier: avoid double notification when it is useless · 0f10851e
    Jérôme Glisse authored
    This patch only affects users of mmu_notifier->invalidate_range callback
    which are device drivers related to ATS/PASID, CAPI, IOMMUv2, SVM ...
    and it is an optimization for those users.  Everyone else is unaffected
    by it.
    
    When clearing a pte/pmd we are given a choice to notify the event under
    the page table lock (notify version of *_clear_flush helpers do call the
    mmu_notifier_invalidate_range).  But that notification is not necessary
    in all cases.
    
    This patch removes almost all cases where it is useless to have a call
    to mmu_notifier_invalidate_range before
    mmu_notifier_invalidate_range_end.  It also adds documentation in all
    those cases explaining why.
    
    Below is a more in depth analysis of why this is fine to do this:
    
    For secondary TLB (non CPU TLB) like IOMMU TLB or device TLB (when
    device use thing like ATS/PASID to get the IOMMU to walk the CPU page
    table to access a process virtual address space).  There is only 2 cases
    when you need to notify those secondary TLB while holding page table
    lock when clearing a pte/pmd:
    
      A) page backing address is free before mmu_notifier_invalidate_range_end
      B) a page table entry is updated to point to a new page (COW, write fault
         on zero page, __replace_page(), ...)
    
    Case A is obvious you do not want to take the risk for the device to write
    to a page that might now be used by something completely different.
    
    Case B is more subtle. For correctness it requires the following sequence
    to happen:
      - take page table lock
      - clear page table entry and notify (pmd/pte_huge_clear_flush_notify())
      - set page table entry to point to new page
    
    If clearing the page table entry is not followed by a notify before setting
    the new pte/pmd value then you can break memory model like C11 or C++11 for
    the device.
    
    Consider the following scenario (device use a feature similar to ATS/
    PASID):
    
    Two address addrA and addrB such that |addrA - addrB| >= PAGE_SIZE we
    assume they are write protected for COW (other case of B apply too).
    
    [Time N] -----------------------------------------------------------------
    CPU-thread-0  {try to write to addrA}
    CPU-thread-1  {try to write to addrB}
    CPU-thread-2  {}
    CPU-thread-3  {}
    DEV-thread-0  {read addrA and populate device TLB}
    DEV-thread-2  {read addrB and populate device TLB}
    [Time N+1] ---------------------------------------------------------------
    CPU-thread-0  {COW_step0: {mmu_notifier_invalidate_range_start(addrA)}}
    CPU-thread-1  {COW_step0: {mmu_notifier_invalidate_range_start(addrB)}}
    CPU-thread-2  {}
    CPU-thread-3  {}
    DEV-thread-0  {}
    DEV-thread-2  {}
    [Time N+2] ---------------------------------------------------------------
    CPU-thread-0  {COW_step1: {update page table point to new page for addrA}}
    CPU-thread-1  {COW_step1: {update page table point to new page for addrB}}
    CPU-thread-2  {}
    CPU-thread-3  {}
    DEV-thread-0  {}
    DEV-thread-2  {}
    [Time N+3] ---------------------------------------------------------------
    CPU-thread-0  {preempted}
    CPU-thread-1  {preempted}
    CPU-thread-2  {write to addrA which is a write to new page}
    CPU-thread-3  {}
    DEV-thread-0  {}
    DEV-thread-2  {}
    [Time N+3] ---------------------------------------------------------------
    CPU-thread-0  {preempted}
    CPU-thread-1  {preempted}
    CPU-thread-2  {}
    CPU-thread-3  {write to addrB which is a write to new page}
    DEV-thread-0  {}
    DEV-thread-2  {}
    [Time N+4] ---------------------------------------------------------------
    CPU-thread-0  {preempted}
    CPU-thread-1  {COW_step3: {mmu_notifier_invalidate_range_end(addrB)}}
    CPU-thread-2  {}
    CPU-thread-3  {}
    DEV-thread-0  {}
    DEV-thread-2  {}
    [Time N+5] ---------------------------------------------------------------
    CPU-thread-0  {preempted}
    CPU-thread-1  {}
    CPU-thread-2  {}
    CPU-thread-3  {}
    DEV-thread-0  {read addrA from old page}
    DEV-thread-2  {read addrB from new page}
    
    So here because at time N+2 the clear page table entry was not pair with a
    notification to invalidate the secondary TLB, the device see the new value
    for addrB before seing the new value for addrA.  This break total memory
    ordering for the device.
    
    When changing a pte to write protect or to point to a new write protected
    page with same content (KSM) it is ok to delay invalidate_range callback
    to mmu_notifier_invalidate_range_end() outside the page table lock.  This
    is true even if the thread doing page table update is preempted right
    after releasing page table lock before calling
    mmu_notifier_invalidate_range_end
    
    Thanks to Andrea for thinking of a problematic scenario for COW.
    
    [jglisse@redhat.com: v2]
      Link: http://lkml.kernel.org/r/20171017031003.7481-2-jglisse@redhat.com
    Link: http://lkml.kernel.org/r/20170901173011.10745-1-jglisse@redhat.comSigned-off-by: 's avatarJérôme Glisse <jglisse@redhat.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Nadav Amit <nadav.amit@gmail.com>
    Cc: Joerg Roedel <jroedel@suse.de>
    Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
    Cc: David Woodhouse <dwmw2@infradead.org>
    Cc: Alistair Popple <alistair@popple.id.au>
    Cc: Michael Ellerman <mpe@ellerman.id.au>
    Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Cc: Stephen Rothwell <sfr@canb.auug.org.au>
    Cc: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
    Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    0f10851e
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