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@xxxxxxxxxxxxxxx>
> ---
> 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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