linux-kernel - Re: [PATCH 3/3] dax: Handle write faults more efficiently

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Date:	Tue, 26 Jan 2016 23:17:06 -0500
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 3/3] dax: Handle write faults more efficiently

On Mon, Jan 25, 2016 at 09:38:19AM -0800, Andy Lutomirski wrote:
> On Mon, Jan 25, 2016 at 9:25 AM, Matthew Wilcox
> <matthew.r.wilcox@...el.com> wrote:
> > From: Matthew Wilcox <willy@...ux.intel.com>
> >
> > 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.