mm,fs,dax: change ->pmd_fault to ->huge_fault

Patch series "1G transparent hugepage support for device dax", v2.

The following series implements support for 1G trasparent hugepage on
x86 for device dax.  The bulk of the code was written by Mathew Wilcox a
while back supporting transparent 1G hugepage for fs DAX.  I have
forward ported the relevant bits to 4.10-rc.  The current submission has
only the necessary code to support device DAX.

Comments from Dan Williams: So the motivation and intended user of this
functionality mirrors the motivation and users of 1GB page support in
hugetlbfs.  Given expected capacities of persistent memory devices an
in-memory database may want to reduce tlb pressure beyond what they can
already achieve with 2MB mappings of a device-dax file.  We have
customer feedback to that effect as Willy mentioned in his previous
version of these patches [1].

[1]: https://lkml.org/lkml/2016/1/31/52

Comments from Nilesh @ Oracle:

There are applications which have a process model; and if you assume
10,000 processes attempting to mmap all the 6TB memory available on a
server; we are looking at the following:

processes         : 10,000
memory            :    6TB
pte @ 4k page size: 8 bytes / 4K of memory * #processes = 6TB / 4k * 8 * 10000 = 1.5GB * 80000 = 120,000GB
pmd @ 2M page size: 120,000 / 512 = ~240GB
pud @ 1G page size: 240GB / 512 = ~480MB

As you can see with 2M pages, this system will use up an exorbitant
amount of DRAM to hold the page tables; but the 1G pages finally brings
it down to a reasonable level.  Memory sizes will keep increasing; so
this number will keep increasing.

An argument can be made to convert the applications from process model
to thread model, but in the real world that may not be always practical.
Hopefully this helps explain the use case where this is valuable.

This patch (of 3):

In preparation for adding the ability to handle PUD pages, convert
vm_operations_struct.pmd_fault to vm_operations_struct.huge_fault.  The
vm_fault structure is extended to include a union of the different page
table pointers that may be needed, and three flag bits are reserved to
indicate which type of pointer is in the union.

[[email protected]: remove unused function ext4_dax_huge_fault()]
  Link: http://lkml.kernel.org/r/[email protected]
[[email protected]: clear PMD or PUD size flags when in fall through path]
  Link: http://lkml.kernel.org/r/148589842696.5820.16078080610311444794.stgit@djiang5-desk3.ch.intel.com
Link: http://lkml.kernel.org/r/148545058784.17912.6353162518188733642.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Matthew Wilcox <[email protected]>
Signed-off-by: Dave Jiang <[email protected]>
Signed-off-by: Ross Zwisler <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Nilesh Choudhury <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Dave Jiang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/ext4/file.c

@@ -273,27 +273,6 @@ static int ext4_dax_fault(struct vm_fault *vmf)
 	return result;
 }
 
-static int
-ext4_dax_pmd_fault(struct vm_fault *vmf)
-{
-	int result;
-	struct inode *inode = file_inode(vmf->vma->vm_file);
-	struct super_block *sb = inode->i_sb;
-	bool write = vmf->flags & FAULT_FLAG_WRITE;
-
-	if (write) {
-		sb_start_pagefault(sb);
-		file_update_time(vmf->vma->vm_file);
-	}
-	down_read(&EXT4_I(inode)->i_mmap_sem);
-	result = dax_iomap_pmd_fault(vmf, &ext4_iomap_ops);
-	up_read(&EXT4_I(inode)->i_mmap_sem);
-	if (write)
-		sb_end_pagefault(sb);
-
-	return result;
-}
-
 /*
  * Handle write fault for VM_MIXEDMAP mappings. Similarly to ext4_dax_fault()
  * handler we check for races agaist truncate. Note that since we cycle through
@@ -326,7 +305,7 @@ static int ext4_dax_pfn_mkwrite(struct vm_fault *vmf)
 
 static const struct vm_operations_struct ext4_dax_vm_ops = {
 	.fault		= ext4_dax_fault,
-	.pmd_fault	= ext4_dax_pmd_fault,
+	.huge_fault	= ext4_dax_fault,
 	.page_mkwrite	= ext4_dax_fault,
 	.pfn_mkwrite	= ext4_dax_pfn_mkwrite,
 };