• Sagi Grimberg's avatar
    mm: move all mmu notifier invocations to be done outside the PT lock · 2ec74c3e
    Sagi Grimberg authored
    In order to allow sleeping during mmu notifier calls, we need to avoid
    invoking them under the page table spinlock.  This patch solves the
    problem by calling invalidate_page notification after releasing the lock
    (but before freeing the page itself), or by wrapping the page invalidation
    with calls to invalidate_range_begin and invalidate_range_end.
    
    To prevent accidental changes to the invalidate_range_end arguments after
    the call to invalidate_range_begin, the patch introduces a convention of
    saving the arguments in consistently named locals:
    
    	unsigned long mmun_start;	/* For mmu_notifiers */
    	unsigned long mmun_end;	/* For mmu_notifiers */
    
    	...
    
    	mmun_start = ...
    	mmun_end = ...
    	mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
    
    	...
    
    	mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
    
    The patch changes code to use this convention for all calls to
    mmu_notifier_invalidate_range_start/end, except those where the calls are
    close enough so that anyone who glances at the code can see the values
    aren't changing.
    
    This patchset is a preliminary step towards on-demand paging design to be
    added to the RDMA stack.
    
    Why do we want on-demand paging for Infiniband?
    
      Applications register memory with an RDMA adapter using system calls,
      and subsequently post IO operations that refer to the corresponding
      virtual addresses directly to HW.  Until now, this was achieved by
      pinning the memory during the registration calls.  The goal of on demand
      paging is to avoid pinning the pages of registered memory regions (MRs).
       This will allow users the same flexibility they get when swapping any
      other part of their processes address spaces.  Instead of requiring the
      entire MR to fit in physical memory, we can allow the MR to be larger,
      and only fit the current working set in physical memory.
    
    Why should anyone care?  What problems are users currently experiencing?
    
      This can make programming with RDMA much simpler.  Today, developers
      that are working with more data than their RAM can hold need either to
      deregister and reregister memory regions throughout their process's
      life, or keep a single memory region and copy the data to it.  On demand
      paging will allow these developers to register a single MR at the
      beginning of their process's life, and let the operating system manage
      which pages needs to be fetched at a given time.  In the future, we
      might be able to provide a single memory access key for each process
      that would provide the entire process's address as one large memory
      region, and the developers wouldn't need to register memory regions at
      all.
    
    Is there any prospect that any other subsystems will utilise these
    infrastructural changes?  If so, which and how, etc?
    
      As for other subsystems, I understand that XPMEM wanted to sleep in
      MMU notifiers, as Christoph Lameter wrote at
      http://lkml.indiana.edu/hypermail/linux/kernel/0802.1/0460.html and
      perhaps Andrea knows about other use cases.
    
      Scheduling in mmu notifications is required since we need to sync the
      hardware with the secondary page tables change.  A TLB flush of an IO
      device is inherently slower than a CPU TLB flush, so our design works by
      sending the invalidation request to the device, and waiting for an
      interrupt before exiting the mmu notifier handler.
    
    Avi said:
    
      kvm may be a buyer.  kvm::mmu_lock, which serializes guest page
      faults, also protects long operations such as destroying large ranges.
      It would be good to convert it into a spinlock, but as it is used inside
      mmu notifiers, this cannot be done.
    
      (there are alternatives, such as keeping the spinlock and using a
      generation counter to do the teardown in O(1), which is what the "may"
      is doing up there).
    
    [akpm@linux-foundation.orgpossible speed tweak in hugetlb_cow(), cleanups]
    Signed-off-by: default avatarAndrea Arcangeli <andrea@qumranet.com>
    Signed-off-by: default avatarSagi Grimberg <sagig@mellanox.com>
    Signed-off-by: default avatarHaggai Eran <haggaie@mellanox.com>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
    Cc: Or Gerlitz <ogerlitz@mellanox.com>
    Cc: Haggai Eran <haggaie@mellanox.com>
    Cc: Shachar Raindel <raindel@mellanox.com>
    Cc: Liran Liss <liranl@mellanox.com>
    Cc: Christoph Lameter <cl@linux-foundation.org>
    Cc: Avi Kivity <avi@redhat.com>
    Cc: Hugh Dickins <hughd@google.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    2ec74c3e
mremap.c 14.3 KB