linux-kernel - Re: [PATCH v7 07/22] Replace the XIP page fault handler with the DAX page fault handler

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Message-ID: <20140811141308.GZ6754@linux.intel.com>
Date:	Mon, 11 Aug 2014 10:13:08 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 07/22] Replace the XIP page fault handler with the DAX
 page fault handler

On Mon, Aug 11, 2014 at 10:51:47AM +0200, Jan Kara wrote:
> So I'm afraid we'll have to find some other way to synchronize
> page faults and truncate / punch hole in DAX.

What if we don't?  If we hit the race (which is vanishingly unlikely with
real applications), the consequence is simply that after a truncate, a
file may be left with one or two blocks allocated somewhere after i_size.
As I understand it, that's not a real problem; they're temporarily
unavailable for allocation but will be freed on file removal or the next
truncation of that file.

I'm also still considering the possibility of having truncate-down block
until all mmaps that extend after the new i_size have been removed ...
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