linux-ext4 - Re: [PATCH v3 0/3] Add XIP support to ext4

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Message-ID: <20131219054303.GA4391@thunk.org>
Date:	Thu, 19 Dec 2013 00:43:03 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Dave Chinner <david@...morbit.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v3 0/3] Add XIP support to ext4

On Wed, Dec 18, 2013 at 09:12:41PM -0700, Matthew Wilcox wrote:
> Now that I've spent the best part of a day looking at the ext4 code, I
> still don't think there's a problem here.  With the way the XIP code is
> currently written (calling ext4_get_block with create=1), we won't get an
> uninitialised extent in the caller.  Instead, we'll get one that's been
> zeroed (the zeroing is part of patch 3/3 and done only for xip files).

If the block was originally allocated via fallocate(2), it will be
marked as uninitialized.  When you call ext4_get_block(), if the block
has been allocated, it will be returned --- and ext4_map_block() as
called by ext4_get_block() does ****not*** clear the uninitialized
flag.  It can't do so because it would be racy; you can only clear the
flag once the data blocks has been written.

As far as patch 3/3, it clears the pages in the page cache, but it
doesn't zap them in the XIP storage device.  But it only does this on
the code path when it allocated a block.  But if the block has already
been previously mapped via fallocate(2), you never hit this section of
code.

Regards,

					- Ted
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