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

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Message-ID: <CALCETrWuPa2SoUcMCtDiv1UDodNqKcQzsZV5PxQx5Xhb524f7w@mail.gmail.com>
Date:	Mon, 25 Jan 2016 09:38:19 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Matthew Wilcox <matthew.r.wilcox@...el.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Matthew Wilcox <willy@...ux.intel.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 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.)

--Andy