Add hooks for disabling direct reclaim

The port of XFS to Linux introduced a thread-specific PF_FSTRANS bit
that is used to mark contexts which are processing transactions.  When
set, allocations in this context can dip into kernel memory reserves
to avoid deadlocks during writeback.  Linux 3.9 provided the additional
PF_MEMALLOC_NOIO for disabling __GFP_IO in page allocations, which XFS
began using in 3.15.

This patch implements hooks for marking transactions via PF_FSTRANS.
When an allocation is performed in the context of PF_FSTRANS, any
KM_SLEEP allocation is transparently converted to a GFP_NOIO allocation.

Additionally, when using a Linux 3.9 or newer kernel, it will set
PF_MEMALLOC_NOIO to prevent direct reclaim from entering pageout() on
on any KM_PUSHPAGE or KM_NOSLEEP allocation.  This effectively allows
the spl_vmalloc() helper function to be used safely in a thread which
is responsible for IO.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

include/sys/kmem.h

@@ -25,6 +25,7 @@
 #ifndef _SPL_KMEM_H
 #define	_SPL_KMEM_H
 
+#include <sys/debug.h>
 #include <linux/slab.h>
 #include <linux/sched.h>
 
@@ -72,6 +73,39 @@ kmem_flags_convert(int flags)
 	return (lflags);
 }
 
+typedef struct {
+	struct task_struct *fstrans_thread;
+	unsigned int saved_flags;
+} fstrans_cookie_t;
+
+static inline fstrans_cookie_t
+spl_fstrans_mark(void)
+{
+	fstrans_cookie_t cookie;
+
+	cookie.fstrans_thread = current;
+	cookie.saved_flags = current->flags & PF_FSTRANS;
+	current->flags |= PF_FSTRANS;
+
+	return (cookie);
+}
+
+static inline void
+spl_fstrans_unmark(fstrans_cookie_t cookie)
+{
+	ASSERT3P(cookie.fstrans_thread, ==, current);
+	ASSERT(current->flags & PF_FSTRANS);
+
+	current->flags &= ~(PF_FSTRANS);
+	current->flags |= cookie.saved_flags;
+}
+
+static inline int
+spl_fstrans_check(void)
+{
+	return (current->flags & PF_FSTRANS);
+}
+
 #ifdef HAVE_ATOMIC64_T
 #define	kmem_alloc_used_add(size)	atomic64_add(size, &kmem_alloc_used)
 #define	kmem_alloc_used_sub(size)	atomic64_sub(size, &kmem_alloc_used)

include/sys/vmem.h

@@ -36,6 +36,7 @@ extern vmem_t *zio_alloc_arena;
 extern vmem_t *zio_arena;
 
 extern size_t vmem_size(vmem_t *vmp, int typemask);
+extern void *spl_vmalloc(unsigned long size, gfp_t lflags, pgprot_t prot);
 
 /*
  * Memory allocation interfaces