linux-ext4 - Re: [PATCH 0/9 v3] ext4: Punch hole and DAX fixes

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Date:	Wed, 4 Nov 2015 11:51:02 -0700
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Jan Kara <jack@...e.com>
Cc:	Ted Tso <tytso@....edu>, linux-ext4@...r.kernel.org,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	dan.j.williams@...el.com
Subject: Re: [PATCH 0/9 v3] ext4: Punch hole and DAX fixes

On Wed, Nov 04, 2015 at 05:18:31PM +0100, Jan Kara wrote:
> Hello,
> 
> Another version of my ext4 fixes. I've fixed up all the failures Ted reported
> except for ext4/001 failures which are false positive (will send fixes for that
> test shortly) and generic/269 in nodelalloc mode which I just wasn't able to
> reproduce.
> 
> Note that testing with 1 KB blocksize on ramdisk is broken since brd has
> buggy discard implementation. It took me quite some time to figure this out.
> Fix is submitted but bear this in mind just in case.
> 
> Changes since v2:
> * Fixed collaps range to truncate pagecache properly with blocksize < pagesize
> * Fixed assertion in ext4_get_blocks_overwrite
> 
> Patch set description
> 
> This series fixes a long standing problem of racing punch hole and page fault
> resulting in possible filesystem corruption or stale data exposure. We fix the
> problem by using a new inode-private rw_semaphore i_mmap_sem to synchronize
> page faults with truncate and punch hole operations.
> 
> When having this exclusion, the only remaining problem with DAX implementation
> are races between two page faults zeroing out same block concurrently (where
> the data written after the first fault finishes are possibly overwritten by
> the second fault still doing zeroing).
> 
> Patch 1 introduces i_mmap_sem lock in ext4 inode and uses it to properly
> serialize extent manipulation operations and page faults.
> 
> Patch 2 is mostly a preparatory cleanup patch which also avoids double lock /
> unlock in unlocked DIO protections (currently harmless but nasty surprise).
> 
> Patches 3-4 fix further races of extent manipulation functions (such as zero
> range, collapse range, insert range) with buffered IO, page writeback
> 
> Patch 5 documents locking order of ext4 filesystem locks.
> 
> Patch 6 removes locking abuse of i_data_sem from the get_blocks() path when
> dioread_nolock is enabled since it is not needed anymore.
> 
> Patches 7-9 implement allocation of pre-zeroed blocks in ext4_map_blocks()
> callback and use such blocks for allocations from DAX page faults.
> 
> The patches survived xfstests run both in dax and non-dax mode.
> 
> 								Honza

This passed all my testing as well.

Tested-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
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