Re: [PATCH 2/3] xfs: consolidate the various page fault handlers

On Thu, Aug 24, 2017 at 05:22:06PM +0200, Christoph Hellwig wrote:

Add a new __xfs_filemap_fault helper that implements all four page fault
callouts, and make these methods themselves small stubs that set the
correct write_fault flag, and exit early for the non-DAX case for the
hugepage related ones.

Life would be so much simpler if we only had one method for all this.

Signed-off-by: Christoph Hellwig <hch@lst.de>

A few nits and a question below.  This looks much simpler and gets rid of a
lot of duplication.

Reviewed-by: Ross Zwisler <redacted>

<>

 STATIC int
 xfs_filemap_huge_fault(
 	struct vm_fault		*vmf,
 	enum page_entry_size	pe_size)
 {
-	struct inode		*inode = file_inode(vmf->vma->vm_file);
-	struct xfs_inode	*ip = XFS_I(inode);
-	int			ret;
-
-	if (!IS_DAX(inode))
+	if (!IS_DAX(file_inode(vmf->vma->vm_file)))
 		return VM_FAULT_FALLBACK;
 
-	trace_xfs_filemap_huge_fault(ip);
-
-	if (vmf->flags & FAULT_FLAG_WRITE) {
-		sb_start_pagefault(inode->i_sb);
-		file_update_time(vmf->vma->vm_file);
-	}
-
-	xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
-	ret = dax_iomap_fault(vmf, pe_size, NULL, &xfs_iomap_ops);
-	xfs_iunlock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
-
-	if (vmf->flags & FAULT_FLAG_WRITE)
-		sb_end_pagefault(inode->i_sb);
+	/* DAX can shortcut the normal fault path on write faults! */

Does this comment still make sense for the PMD case?  Here's what I *think* it
means: For DAX write faults we make the PTE writeable in the ->fault() code
and don't rely on a follow-up ->page_mkwrite() call.  This happens in
do_shared_fault(), where we return before the ->page_mkwrite() call because
the DAX write fault returns VM_FAULT_NOPAGE.

This is in contrast to the normal page fault case where the ->fault() call
ends up calling filemap_fault(), which populates the page read-only, and then
do_shared_fault() does a follow-up ->page_mkwrite() call which makes the page
writable.

First, is the above correct?  :)  If so, I think the comment doesn't make
sense for the PMD fault path because in that case we always make the PMD
writeable in the first ->huge_fault() call, as there is no follow-up
*_mkwrite()?

+	return __xfs_filemap_fault(vmf, pe_size,
+			(vmf->flags & FAULT_FLAG_WRITE));
+}
 
-	return ret;
+STATIC int
+xfs_filemap_page_mkwrite(
+	struct vm_fault		*vmf)
+{
+	return __xfs_filemap_fault(vmf, PE_SIZE_PTE, true);
 }
 
-/*
- * pfn_mkwrite was originally inteneded to ensure we capture time stamp
- * updates on write faults. In reality, it's need to serialise against
- * truncate similar to page_mkwrite. Hence we cycle the XFS_MMAPLOCK_SHARED
- * to ensure we serialise the fault barrier in place.
- */
 static int

   STATIC

 xfs_filemap_pfn_mkwrite(
 	struct vm_fault		*vmf)
 {
-
-	struct inode		*inode = file_inode(vmf->vma->vm_file);
-	struct xfs_inode	*ip = XFS_I(inode);
-	int			ret = VM_FAULT_NOPAGE;
-	loff_t			size;
-
-	trace_xfs_filemap_pfn_mkwrite(ip);
-
-	sb_start_pagefault(inode->i_sb);
-	file_update_time(vmf->vma->vm_file);
-
-	/* check if the faulting page hasn't raced with truncate */
-	xfs_ilock(ip, XFS_MMAPLOCK_SHARED);
-	size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;

I see that this size check is gone from the new code, and we now rely on the
equivalent check in dax_iomap_fault().  Nice.

I wonder if pe_size would be more readable as a string via __print_symbolic()
so we see "pe_size PMD" instead of "pe_size 1" from enum page_entry_size?