linux-ext4 - Re: [PATCH v2 2/9] ext2: tell DAX the size of allocation holes

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Date:   Thu, 15 Sep 2016 14:09:28 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Jan Kara <jack@...e.cz>
Cc:     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,
        Matthew Wilcox <mawilcox@...rosoft.com>
Subject: Re: [PATCH v2 2/9] ext2: tell DAX the size of allocation holes

On Tue, Aug 23, 2016 at 04:04:12PM -0600, 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.
> 
> Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> ---
>  fs/ext2/inode.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> index d5c7d09..dd55d74 100644
> --- a/fs/ext2/inode.c
> +++ b/fs/ext2/inode.c
> @@ -773,6 +773,9 @@ 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) {
> +		/* hole case, need to fill in bh_result->b_size */
> +		bh_result->b_size = 1 << inode->i_blkbits;
>  	}
>  	return ret;
>  
> -- 
> 2.9.0
> 

Jan, is it possible for ext2 to return 2 MiB of contiguous space to us via
ext2_get_block()?

I ask because we have all the infrastructure in place for ext2 to handle PMD
faults (ext2_dax_pmd_fault(), etc.), but I don't think in my testing I've ever
seen this actually happen.

ext2 can obviously return multiple blocks from ext2_get_block(), but can it
actually satisfy a whole PMD's worth (512 contiguous blocks)?  If so, what
steps do I need to take to get this to work in my testing?

If it can't happen, we should probably rip out ext2_dax_pmd_fault() so that we
don't have to keep falling back to PTEs via the PMD path.

Thanks,
- Ross
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