linux-ext4 - Re: [PATCH 2/2] ext2: Add locking for DAX faults

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Date:	Fri, 9 Oct 2015 16:38:02 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
	Dave Chinner <david@...morbit.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Andreas Dilger <andreas.dilger@...el.com>
Subject: Re: [PATCH 2/2] ext2: Add locking for DAX faults

On Fri, Oct 09, 2015 at 03:18:11PM -0700, Dan Williams wrote:
> On Fri, Oct 9, 2015 at 3:02 PM, Ross Zwisler
> <ross.zwisler@...ux.intel.com> wrote:
> > Add locking to ensure that DAX faults are isolated from ext2 operations
> > that modify the data blocks allocation for an inode.  This is intended to
> > be analogous to the work being done in XFS by Dave Chinner:
> >
> > http://www.spinics.net/lists/linux-fsdevel/msg90260.html
> >
> > Compared with XFS the ext2 case is greatly simplified by the fact that ext2
> > already allocates and zeros new blocks before they are returned as part of
> > ext2_get_block(), so DAX doesn't need to worry about getting unmapped or
> > unwritten buffer heads.
> >
> > This means that the only work we need to do in ext2 is to isolate the DAX
> > faults from inode block allocation changes.  I believe this just means that
> > we need to isolate the DAX faults from truncate operations.
> >
> > The newly introduced dax_sem is intended to replicate the protection
> > offered by i_mmaplock in XFS.  In addition to truncate the i_mmaplock also
> > protects XFS operations like hole punching, fallocate down, extent
> > manipulation IOCTLS like xfs_ioc_space() and extent swapping.  Truncate is
> > the only one of these operations supported by ext2.
> >
> > Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> [..]
> 
> ...not a review of the ext2 changes.
> 
> > diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> > index c60a248..2b974fc 100644
> > --- a/fs/ext2/inode.c
> > +++ b/fs/ext2/inode.c
> > @@ -1085,6 +1085,7 @@ static void ext2_free_branches(struct inode *inode, __le32 *p, __le32 *q, int de
> >                 ext2_free_data(inode, p, q);
> >  }
> >
> > +/* dax_sem must be held when calling this function */
> >  static void __ext2_truncate_blocks(struct inode *inode, loff_t offset)
> >  {
> 
> How about  a "WARN_ON(!rwsem_is_locked(&ei->dax_sem));" to backstop
> this assumption?

Yep, sounds like a good idea.  Thanks.
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