linux-kernel - Re: [PATCH 1/7] ext2: tell DAX the size of allocation holes

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Date:	Tue, 16 Aug 2016 16:52:27 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-nvdimm@...ts.01.org
Subject: Re: [PATCH 1/7] ext2: tell DAX the size of allocation holes

On Tue, Aug 16, 2016 at 11:10:25AM +0200, Jan Kara wrote:
> On Mon 15-08-16 13:09:12, Ross Zwisler wrote:
> > When DAX calls ext2_get_block() and the file offset points to a hole we
> > currently don't set bh_result->b_size.  When we re-enable PMD faults DAX
> > will need bh_result->b_size to tell it the size of the hole so it can
> > decide whether to fault in a 4 KiB zero page or a 2 MiB zero page.
> > 
> > For ext2 we always want DAX to use 4 KiB zero pages, so we just tell DAX
> > that all holes are 4 KiB in size.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> > ---
> >  fs/ext2/inode.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> > index d5c7d09..c6d9763 100644
> > --- a/fs/ext2/inode.c
> > +++ b/fs/ext2/inode.c
> > @@ -773,6 +773,12 @@ int ext2_get_block(struct inode *inode, sector_t iblock, struct buffer_head *bh_
> >  	if (ret > 0) {
> >  		bh_result->b_size = (ret << inode->i_blkbits);
> >  		ret = 0;
> > +	} else if (ret == 0 && IS_DAX(inode)) {
> 
> I'd just drop the IS_DAX() check and set
> 
> 	bh_result->b_size = 1 << inode->i_blkbits;
> 
> IMO it's better to have things consistent between DAX & !DAX whenever
> possible.

Agreed, this is better.  Fixed for v2, thanks!