linux-ext4 - Re: [PATCH 0/9 v4] ext4: Punch hole and DAX fixes

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Date:	Wed, 18 Nov 2015 16:13:02 +0100
From:	Jan Kara <jack@...e.cz>
To:	"Boylston, Brian" <brian.boylston@....com>
Cc:	Jan Kara <jack@...e.com>, Ted Tso <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>
Subject: Re: [PATCH 0/9 v4] ext4: Punch hole and DAX fixes

On Tue 17-11-15 17:41:55, Boylston, Brian wrote:
> On Tue, Nov 10, 2015 at 2:51 PM, Jan Kara wrote:
> > submitted. Also note that testing with 1 KB blocksize on ramdisk is broken
> > since brd has buggy discard implementation - Jens has a fix queued.
> > 
> > Change since v3:
> > * Fixed ext4_dax_mmap_get_block() to not return buffer_new buffer and thus
> >   avoid racy zeroing in generic dax code
> > * Fixed ext4_map_blocks() to zeroout blocks before inserting entry into
> >   extent status tree to avoid racy lookups of blocks.
> > 
> > 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).
> 
> Is this still a problem for this version of the patch set?

No, the patch set fixes this problem. The paragraph meant to say that first
few patches fix hole punching issues and then a few patches fix this DAX
issue. But the wording is strange, I agree.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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