linux-ext4 - Re: [PATCH 2/7] dax: Make huge page handling depend of CONFIG

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Date:	Thu, 12 May 2016 12:59:58 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-nvdimm@...ts.01.org, Ted Tso <tytso@....edu>,
	linux-ext4@...r.kernel.org,
	Vishal Verma <vishal.l.verma@...el.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/7] dax: Make huge page handling depend of CONFIG_BROKEN

On Thu, May 12, 2016 at 06:29:15PM +0200, Jan Kara wrote:
> Currently the handling of huge pages for DAX is racy. For example the
> following can happen:
> 
> CPU0 (THP write fault)			CPU1 (normal read fault)
> 
> __dax_pmd_fault()			__dax_fault()
>   get_block(inode, block, &bh, 0) -> not mapped
> 					get_block(inode, block, &bh, 0)
> 					  -> not mapped
>   if (!buffer_mapped(&bh) && write)
>     get_block(inode, block, &bh, 1) -> allocates blocks
>   truncate_pagecache_range(inode, lstart, lend);
> 					dax_load_hole();
> 
> This results in data corruption since process on CPU1 won't see changes
> into the file done by CPU0.
> 
> The race can happen even if two normal faults race however with THP the
> situation is even worse because the two faults don't operate on the same
> entries in the radix tree and we want to use these entries for
> serialization. So make THP support in DAX code depend on CONFIG_BROKEN
> for now.
> 
> Signed-off-by: Jan Kara <jack@...e.cz>

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