Re: [PATCH v3 02/15] dax: increase granularity of dax_clear_blocks() operations

[Dave Chinner <david@fromorbit.com>]

On Sun, Nov 01, 2015 at 11:29:53PM -0500, Dan Williams wrote:
> dax_clear_blocks is currently performing a cond_resched() after every
> PAGE_SIZE memset.  We need not check so frequently, for example md-raid
> only calls cond_resched() at stripe granularity.  Also, in preparation
> for introducing a dax_map_atomic() operation that temporarily pins a dax
> mapping move the call to cond_resched() to the outer loop.
> 
> Reviewed-by: Jan Kara <jack@suse.com>
> Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  fs/dax.c |   27 ++++++++++++---------------
>  1 file changed, 12 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 5dc33d788d50..f8e543839e5c 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -28,6 +28,7 @@
>  #include <linux/sched.h>
>  #include <linux/uio.h>
>  #include <linux/vmstat.h>
> +#include <linux/sizes.h>
>  
>  int dax_clear_blocks(struct inode *inode, sector_t block, long size)
>  {
> @@ -38,24 +39,20 @@ int dax_clear_blocks(struct inode *inode, sector_t block, long size)
>  	do {
>  		void __pmem *addr;
>  		unsigned long pfn;
> -		long count;
> +		long count, sz;
>  
> -		count = bdev_direct_access(bdev, sector, &addr, &pfn, size);
> +		sz = min_t(long, size, SZ_1M);
> +		count = bdev_direct_access(bdev, sector, &addr, &pfn, sz);
>  		if (count < 0)
>  			return count;
> -		BUG_ON(size < count);
> -		while (count > 0) {
> -			unsigned pgsz = PAGE_SIZE - offset_in_page(addr);
> -			if (pgsz > count)
> -				pgsz = count;
> -			clear_pmem(addr, pgsz);
> -			addr += pgsz;
> -			size -= pgsz;
> -			count -= pgsz;
> -			BUG_ON(pgsz & 511);
> -			sector += pgsz / 512;
> -			cond_resched();
> -		}
> +		if (count < sz)
> +			sz = count;
> +		clear_pmem(addr, sz);
> +		addr += sz;
> +		size -= sz;
> +		BUG_ON(sz & 511);
> +		sector += sz / 512;
> +		cond_resched();
>  	} while (size);
>  
>  	wmb_pmem();

dax_clear_blocks() needs to go away and be replaced by a driver
level implementation of blkdev_issue_zerout(). This is effectively a
block device operation (we're taking sector addresses and zeroing
them), so it really belongs in the pmem drivers rather than the DAX
code.

I suspect a REQ_WRITE_SAME implementation is the way to go here, as
then the filesystems can just call sb_issue_zerout() and the block
layer zeroing will work on all types of storage without the
filesystem having to care whether DAX is in use or not.

Putting the implementation of the zeroing in the pmem drivers will
enable the drivers to optimise the caching behaviour of block
zeroing.  The synchronous cache flushing behaviour of this function
is a performance killer as we are now block zeroing on allocation
and that results in two synchronous data writes (zero on alloc,
commit, write data, commit) for each page.

The zeroing (and the data, for that matter) doesn't need to be
committed to persistent store until the allocation is written and
committed to the journal - that will happen with a REQ_FLUSH|REQ_FUA
write, so it makes sense to deploy the big hammer and delay the
blocking CPU cache flushes until the last possible moment in cases
like this.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com