dax: add tracepoint infrastructure, PMD tracing

Tracepoints are the standard way to capture debugging and tracing
information in many parts of the kernel, including the XFS and ext4
filesystems.  Create a tracepoint header for FS DAX and add the first DAX
tracepoints to the PMD fault handler.  This allows the tracing for DAX to
be done in the same way as the filesystem tracing so that developers can
look at them together and get a coherent idea of what the system is doing.

I added both an entry and exit tracepoint because future patches will add
tracepoints to child functions of dax_iomap_pmd_fault() like
dax_pmd_load_hole() and dax_pmd_insert_mapping().  We want those messages
to be wrapped by the parent function tracepoints so the code flow is more
easily understood.  Having entry and exit tracepoints for faults also
allows us to easily see what filesystems functions were called during the
fault.  These filesystem functions get executed via iomap_begin() and
iomap_end() calls, for example, and will have their own tracepoints.

For PMD faults we primarily want to understand the type of mapping, the
fault flags, the faulting address and whether it fell back to 4k faults.
If it fell back to 4k faults the tracepoints should let us understand why.

I named the new tracepoint header file "fs_dax.h" to allow for device DAX
to have its own separate tracing header in the same directory at some
point.

Here is an example output for these events from a successful PMD fault:

  big-1441  [005] ....    32.582758: xfs_filemap_pmd_fault: dev 259:0 ino 0x1003

  big-1441  [005] ....    32.582776: dax_pmd_fault: dev 259:0 ino 0x1003
  shared WRITE|ALLOW_RETRY|KILLABLE|USER address 0x10505000 vm_start 0x10200000 vm_end 0x10700000 pgoff 0x200 max_pgoff 0x1400

  big-1441  [005] ....    32.583292: dax_pmd_fault_done: dev 259:0 ino 0x1003
  shared WRITE|ALLOW_RETRY|KILLABLE|USER address 0x10505000 vm_start 0x10200000 vm_end 0x10700000 pgoff 0x200 max_pgoff 0x1400 NOPAGE

Link: http://lkml.kernel.org/r/1484085142-2297-3-git-send-email-ross.zwisler@linux.intel.com
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Suggested-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 20 insertions, 10 deletions

diff --git a/fs/dax.c b/fs/dax.c
index e9cf8b4cd234..dc11a2b90731 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -35,6 +35,9 @@
 #include <linux/iomap.h>
 #include "internal.h"
 
+#define CREATE_TRACE_POINTS
+#include <trace/events/fs_dax.h>
+
 /* We choose 4096 entries - same as per-zone page wait tables */
 #define DAX_WAIT_TABLE_BITS 12
 #define DAX_WAIT_TABLE_ENTRIES (1 << DAX_WAIT_TABLE_BITS)
@@ -1341,6 +1344,16 @@ int dax_iomap_pmd_fault(struct vm_area_struct *vma, unsigned long address,
 	loff_t pos;
 	int error;
 
+	/*
+	 * Check whether offset isn't beyond end of file now. Caller is
+	 * supposed to hold locks serializing us with truncate / punch hole so
+	 * this is a reliable test.
+	 */
+	pgoff = linear_page_index(vma, pmd_addr);
+	max_pgoff = (i_size_read(inode) - 1) >> PAGE_SHIFT;
+
+	trace_dax_pmd_fault(inode, vma, address, flags, pgoff, max_pgoff, 0);
+
 	/* Fall back to PTEs if we're going to COW */
 	if (write && !(vma->vm_flags & VM_SHARED))
 		goto fallback;
@@ -1351,16 +1364,10 @@ int dax_iomap_pmd_fault(struct vm_area_struct *vma, unsigned long address,
 	if ((pmd_addr + PMD_SIZE) > vma->vm_end)
 		goto fallback;
 
-	/*
-	 * Check whether offset isn't beyond end of file now. Caller is
-	 * supposed to hold locks serializing us with truncate / punch hole so
-	 * this is a reliable test.
-	 */
-	pgoff = linear_page_index(vma, pmd_addr);
-	max_pgoff = (i_size_read(inode) - 1) >> PAGE_SHIFT;
-
-	if (pgoff > max_pgoff)
-		return VM_FAULT_SIGBUS;
+	if (pgoff > max_pgoff) {
+		result = VM_FAULT_SIGBUS;
+		goto out;
+	}
 
 	/* If the PMD would extend beyond the file size */
 	if ((pgoff | PG_PMD_COLOUR) > max_pgoff)
@@ -1432,6 +1439,9 @@ int dax_iomap_pmd_fault(struct vm_area_struct *vma, unsigned long address,
 		split_huge_pmd(vma, pmd, address);
 		count_vm_event(THP_FAULT_FALLBACK);
 	}
+out:
+	trace_dax_pmd_fault_done(inode, vma, address, flags, pgoff, max_pgoff,
+			result);
 	return result;
 }
 EXPORT_SYMBOL_GPL(dax_iomap_pmd_fault);