Use KM_PUSHPAGE in l2arc_write_buffers

There is potential for deadlock in the l2arc_feed thread if KM_PUSHPAGE
is not used for the allocations made in l2arc_write_buffers.
Specifically, if KM_PUSHPAGE is not used for these allocations, it is
possible for reclaim to be triggered which can cause the l2arc_feed
thread to deadlock itself on the ARC_mru mutex. An example of this is
demonstrated in the following backtrace of the l2arc_feed thread:

    crash> bt 4123
    PID: 4123   TASK: ffff88062f8c1500  CPU: 6   COMMAND: "l2arc_feed"
      0 [ffff88062511d610] schedule at ffffffff814eeee0
      1 [ffff88062511d6d8] __mutex_lock_slowpath at ffffffff814f057e
      2 [ffff88062511d748] mutex_lock at ffffffff814f041b
      3 [ffff88062511d768] arc_evict at ffffffffa05130ca [zfs]
      4 [ffff88062511d858] arc_adjust at ffffffffa05139a9 [zfs]
      5 [ffff88062511d878] arc_shrink at ffffffffa0513a95 [zfs]
      6 [ffff88062511d898] arc_kmem_reap_now at ffffffffa0513be8 [zfs]
      7 [ffff88062511d8c8] arc_shrinker_func at ffffffffa0513ccc [zfs]
      8 [ffff88062511d8f8] shrink_slab at ffffffff8112a17a
      9 [ffff88062511d958] do_try_to_free_pages at ffffffff8112bfdf
     10 [ffff88062511d9e8] try_to_free_pages at ffffffff8112c3ed
     11 [ffff88062511da98] __alloc_pages_nodemask at ffffffff8112431d
     12 [ffff88062511dbb8] kmem_getpages at ffffffff8115e632
     13 [ffff88062511dbe8] fallback_alloc at ffffffff8115f24a
     14 [ffff88062511dc68] ____cache_alloc_node at ffffffff8115efc9
     15 [ffff88062511dcc8] __kmalloc at ffffffff8115fbf9
     16 [ffff88062511dd18] kmem_alloc_debug at ffffffffa047b8cb [spl]
     17 [ffff88062511dda8] l2arc_feed_thread at ffffffffa0511e71 [zfs]
     18 [ffff88062511dea8] thread_generic_wrapper at ffffffffa047d1a1 [spl]
     19 [ffff88062511dee8] kthread at ffffffff81090a86
     20 [ffff88062511df48] kernel_thread at ffffffff8100c14a

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Integrate ARC more tightly with Linux

Under Solaris the ARC was designed to stay one step ahead of the
VM subsystem.  It would attempt to recognize low memory situtions
before they occured and evict data from the cache.  It would also
make assessments about if there was enough free memory to perform
a specific operation.

This was all possible because Solaris exposes a fairly decent
view of the memory state of the system to other kernel threads.
Linux on the other hand does not make this information easily
available.  To avoid extensive modifications to the ARC the SPL
attempts to provide these same interfaces.  While this works it
is not ideal and problems can arise when the ARC and Linux have
different ideas about when your out of memory.  This has manifested
itself in the past as a spinning arc_reclaim_thread.

This patch abandons the emulated Solaris interfaces in favor of
the prefered Linux interface.  That means moving the bulk of the
memory reclaim logic out of the arc_reclaim_thread and in to the
evict driven shrinker callback.  The Linux VM will call this
function when it needs memory.  The ARC is then responsible for
attempting to free the requested amount of memory if possible.

Several interfaces have been modified to accomidate this approach,
however the basic user space implementation remains the same.
The following changes almost exclusively just apply to the kernel
implementation.

* Removed the hdr_recl() reclaim callback which is redundant
  with the broader arc_shrinker_func().

* Reduced arc_grow_retry to 5 seconds from 60.  This is now used
  internally in the ARC with arc_no_grow to indicate that direct
  reclaim was recently performed.  This typically indicates a
  rapid change in memory demands which the kswapd threads were
  unable to keep ahead of.  As long as direct reclaim is happening
  once every 5 seconds arc growth will be paused to avoid further
  contributing to the existing memory pressure.  The more common
  indirect reclaim paths will not set arc_no_grow.

* arc_shrink() has been extended to take the number of bytes by
  which arc_c should be reduced.  This allows for a more granual
  reduction of the arc target.  Since the kernel provides a
  reclaim value to the arc_shrinker_func() this value is used
  instead of 1<<arc_shrink_shift.

* arc_reclaim_needed() has been removed.  It was used to determine
  if the system was under memory pressure and relied extensively
  on Solaris specific VM interfaces.  In most case the new code
  just checks arc_no_grow which indicates that within the last
  arc_grow_retry seconds direct memory reclaim occurred.

* arc_memory_throttle() has been updated to always include the
  amount of evictable memory (arc and page cache) in its free
  space calculations.  This space is largely available in most
  call paths due to direct memory reclaim.

* The Solaris pageout code was also removed to avoid confusion.
  It has always been disabled due to proc_pageout being defined
  as NULL in the Linux port.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
OS-682: ARC minimum is set too high, potentially blocking VM creation
Update ARC memory limits to account for SLUB internal fragmentation

23bdb07d4e4c435205d25d3efdb5fef2d089ce5e updated the ARC memory limits
to be 1/2 of memory or all but 4GB. Unfortunately, these values assume
zero internal fragmentation in the SLUB allocator, when in reality, the
internal fragmentation could be as high as 50%, effectively doubling
memory usage. This poses clear safety issues, because it permits the
size of ARC to exceed system memory.

This patch changes this to set arc_c_max to 1/3 of system memory, which
effectively limits arc_c_max to [1/3,2/3) when internal fragmentation is
taken into account. The average of this is 1/2 of system memory, which
is much more reasonable than what was possible in the past.
Revert CONFIG_PREEMPT checks
Modify txg_hold_open to support preemptable kernels
Disable direct reclaim on zvols

Previously, it was possible for the direct reclaim path to be invoked
when a write to a zvol was made. When a zvol is used as a swap device,
this often causes swap requests to depend on additional swap requests,
which deadlocks. We address this by disabling the direct reclaim path
on zvols.

This closes issue #83.