mmu-notifiers: add mm_take_all_locks() operation

mm_take_all_locks holds off reclaim from an entire mm_struct.  This allows
mmu notifiers to register into the mm at any time with the guarantee that
no mmu operation is in progress on the mm.

This operation locks against the VM for all pte/vma/mm related operations
that could ever happen on a certain mm.  This includes vmtruncate,
try_to_unmap, and all page faults.

The caller must take the mmap_sem in write mode before calling
mm_take_all_locks().  The caller isn't allowed to release the mmap_sem
until mm_drop_all_locks() returns.

mmap_sem in write mode is required in order to block all operations that
could modify pagetables and free pages without need of altering the vma
layout (for example populate_range() with nonlinear vmas).  It's also
needed in write mode to avoid new anon_vmas to be associated with existing
vmas.

A single task can't take more than one mm_take_all_locks() in a row or it
would deadlock.

mm_take_all_locks() and mm_drop_all_locks are expensive operations that
may have to take thousand of locks.

mm_take_all_locks() can fail if it's interrupted by signals.

When mmu_notifier_register returns, we must be sure that the driver is
notified if some task is in the middle of a vmtruncate for the 'mm' where
the mmu notifier was registered (mmu_notifier_invalidate_range_start/end
is run around the vmtruncation but mmu_notifier_register can run after
mmu_notifier_invalidate_range_start and before
mmu_notifier_invalidate_range_end).  Same problem for rmap paths.  And
we've to remove page pinning to avoid replicating the tlb_gather logic
inside KVM (and GRU doesn't work well with page pinning regardless of
needing tlb_gather), so without mm_take_all_locks when vmtruncate frees
the page, kvm would have no way to notice that it mapped into sptes a page
that is going into the freelist without a chance of any further
mmu_notifier notification.

[[email protected]: coding-style fixes]
Signed-off-by: Andrea Arcangeli <[email protected]>
Acked-by: Linus Torvalds <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Jack Steiner <[email protected]>
Cc: Robin Holt <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Kanoj Sarcar <[email protected]>
Cc: Roland Dreier <[email protected]>
Cc: Steve Wise <[email protected]>
Cc: Avi Kivity <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Rusty Russell <[email protected]>
Cc: Anthony Liguori <[email protected]>
Cc: Chris Wright <[email protected]>
Cc: Marcelo Tosatti <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Izik Eidus <[email protected]>
Cc: Anthony Liguori <[email protected]>
Cc: Rik van Riel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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