2016-08-15 19:09:12

by Ross Zwisler

Subject: [PATCH 1/7] ext2: tell DAX the size of allocation holes

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-VuQAYsv1563Yd54FQh9/[email protected]>
---
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)) {
+ /*
+ * We have hit a hole. Tell DAX it is 4k in size so that it
+ * uses PTE faults.
+ */
+ bh_result->b_size = PAGE_SIZE;
}
return ret;

--
2.9.0

2016-08-16 09:10:25

by Jan Kara

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

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 <[email protected]>
> ---
> 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.

Honza

> + /*
> + * We have hit a hole. Tell DAX it is 4k in size so that it
> + * uses PTE faults.
> + */
> + bh_result->b_size = PAGE_SIZE;
> }
> return ret;
>
> --
> 2.9.0
>
>
--
Jan Kara <[email protected]>
SUSE Labs, CR

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2016-08-16 22:52:27

by Ross Zwisler

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-VuQAYsv1563Yd54FQh9/[email protected]>
> > ---
> > 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!