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

[Ross Zwisler]

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 
> > ---
> >  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!

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

[Ross Zwisler]

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 
> > ---
> >  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!

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

[Jan Kara]

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 
> ---
>  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 
SUSE Labs, CR

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

[Jan Kara]

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 
> ---
>  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 
SUSE Labs, CR