On Mon, Jan 25, 2016 at 09:38:19AM -0800, Andy Lutomirski wrote:

On Mon, Jan 25, 2016 at 9:25 AM, Matthew Wilcox
[off-list ref] wrote:

quoted

From: Matthew Wilcox <redacted>

When we handle a write-fault on a DAX mapping, we currently insert a
read-only mapping and then take the page fault again to convert it to
a writable mapping.  This is necessary for the case where we cover a
hole with a read-only zero page, but when we have a data block already
allocated, it is inefficient.

Use the recently added vmf_insert_pfn_prot() to insert a writable mapping,
even though the default VM flags say to use a read-only mapping.

Conceptually, I like this.  Do you need to make sure to do all the
do_wp_page work, though?  (E.g. we currently update mtime in there.
Some day I'll fix that, but it'll be replaced with a set_bit to force
a deferred mtime update.)

We update mtime in the ->fault handler of filesystems which support DAX
like this:

        if (vmf->flags & FAULT_FLAG_WRITE) {
                sb_start_pagefault(inode->i_sb);
                file_update_time(vma->vm_file);
        }

so I think we're covered.

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