linux-ext4 - Re: [PATCH] ext4: re-enable extent zeroout optimization on encrypted files

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Date:   Mon, 13 Jan 2020 14:35:08 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-ext4@...r.kernel.org, linux-fscrypt@...r.kernel.org
Subject: Re: [PATCH] ext4: re-enable extent zeroout optimization on encrypted
 files

On Thu, Dec 26, 2019 at 10:11:14AM -0600, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> For encrypted files, commit 36086d43f657 ("ext4 crypto: fix bugs in
> ext4_encrypted_zeroout()") disabled the optimization where when a write
> occurs to the middle of an unwritten extent, the head and/or tail of the
> extent (when they aren't too large) are zeroed out, turned into an
> initialized extent, and merged with the part being written to.  This
> optimization helps prevent fragmentation of the extent tree.
> 
> However, disabling this optimization also made fscrypt_zeroout_range()
> nearly impossible to test, as now it's only reachable via the very rare
> case in ext4_split_extent_at() where allocating a new extent tree block
> fails due to ENOSPC.  'gce-xfstests -c ext4/encrypt -g auto' doesn't
> even hit this at all.
> 
> It's preferable to avoid really rare cases that are hard to test.
> 
> That commit also cited data corruption in xfstest generic/127 as a
> reason to disable the extent zeroout optimization, but that's no longer
> reproducible anymore.  It also cited fscrypt_zeroout_range() having poor
> performance, but I've written a patch to fix that.
> 
> Therefore, re-enable the extent zeroout optimization on encrypted files.
> 
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

Thanks, applied.

						- Ted