Re: [PATCH 01/11] Btrfs: __btrfs_buffered_write: Reserve/release extents aligned to block size

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

Hi Chanda,

Thanks for your effort to implement sub pagesize block size.

These cleanups look quite good, but still some small readablity 
recommendation inlined below.

Chandan Rajendra wrote on 2015/08/06 15:40 +0530:
> Currently, the code reserves/releases extents in multiples of PAGE_CACHE_SIZE
> units. Fix this by doing reservation/releases in block size units.
>
> Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
> ---
>   fs/btrfs/file.c | 40 ++++++++++++++++++++++++++++------------
>   1 file changed, 28 insertions(+), 12 deletions(-)
>
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 795d754..e3b2b3c 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -1362,16 +1362,19 @@ fail:
>   static noinline int
>   lock_and_cleanup_extent_if_need(struct inode *inode, struct page **pages,
>   				size_t num_pages, loff_t pos,
> +				size_t write_bytes,
>   				u64 *lockstart, u64 *lockend,
>   				struct extent_state **cached_state)
>   {
> +	struct btrfs_root *root = BTRFS_I(inode)->root;
>   	u64 start_pos;
>   	u64 last_pos;
>   	int i;
>   	int ret = 0;
>
> -	start_pos = pos & ~((u64)PAGE_CACHE_SIZE - 1);
> -	last_pos = start_pos + ((u64)num_pages << PAGE_CACHE_SHIFT) - 1;
> +	start_pos = pos & ~((u64)root->sectorsize - 1);
Why not just roundown(pos, root->sectorisze)
Hard coded align is never that easy to read.

> +	last_pos = start_pos
> +		+ ALIGN(pos + write_bytes - start_pos, root->sectorsize) - 1;
Maybe just a preference problem, I'd prefer to use round_down other than
ALIGN, as sometimes I still need to figure out if it is round_down or 
round_down.
>
>   	if (start_pos < inode->i_size) {
>   		struct btrfs_ordered_extent *ordered;
> @@ -1489,6 +1492,7 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
>
>   	while (iov_iter_count(i) > 0) {
>   		size_t offset = pos & (PAGE_CACHE_SIZE - 1);
> +		size_t sector_offset;
>   		size_t write_bytes = min(iov_iter_count(i),
>   					 nrptrs * (size_t)PAGE_CACHE_SIZE -
>   					 offset);
> @@ -1497,6 +1501,8 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
>   		size_t reserve_bytes;
>   		size_t dirty_pages;
>   		size_t copied;
> +		size_t dirty_sectors;
> +		size_t num_sectors;
>
>   		WARN_ON(num_pages > nrptrs);
>
> @@ -1509,8 +1515,12 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
>   			break;
>   		}
>
> -		reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
> +		sector_offset = pos & (root->sectorsize - 1);
Same here.

Thanks,
Qu
> +		reserve_bytes = ALIGN(write_bytes + sector_offset,
> +				root->sectorsize);
> +
>   		ret = btrfs_check_data_free_space(inode, reserve_bytes, write_bytes);
> +
>   		if (ret == -ENOSPC &&
>   		    (BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
>   					      BTRFS_INODE_PREALLOC))) {
> @@ -1523,7 +1533,9 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
>   				 */
>   				num_pages = DIV_ROUND_UP(write_bytes + offset,
>   							 PAGE_CACHE_SIZE);
> -				reserve_bytes = num_pages << PAGE_CACHE_SHIFT;
> +				reserve_bytes = ALIGN(write_bytes + sector_offset,
> +						root->sectorsize);
> +
>   				ret = 0;
>   			} else {
>   				ret = -ENOSPC;
> @@ -1558,8 +1570,8 @@ again:
>   			break;
>
>   		ret = lock_and_cleanup_extent_if_need(inode, pages, num_pages,
> -						      pos, &lockstart, &lockend,
> -						      &cached_state);
> +						pos, write_bytes, &lockstart,
> +						&lockend, &cached_state);
>   		if (ret < 0) {
>   			if (ret == -EAGAIN)
>   				goto again;
> @@ -1595,9 +1607,14 @@ again:
>   		 * we still have an outstanding extent for the chunk we actually
>   		 * managed to copy.
>   		 */
> -		if (num_pages > dirty_pages) {
> -			release_bytes = (num_pages - dirty_pages) <<
> -				PAGE_CACHE_SHIFT;
> +		num_sectors = reserve_bytes >> inode->i_blkbits;
> +		dirty_sectors = round_up(copied + sector_offset,
> +					root->sectorsize);
> +		dirty_sectors >>= inode->i_blkbits;
> +
> +		if (num_sectors > dirty_sectors) {
> +			release_bytes = (write_bytes - copied)
> +				& ~((u64)root->sectorsize - 1);
>   			if (copied > 0) {
>   				spin_lock(&BTRFS_I(inode)->lock);
>   				BTRFS_I(inode)->outstanding_extents++;
> @@ -1611,7 +1628,7 @@ again:
>   							     release_bytes);
>   		}
>
> -		release_bytes = dirty_pages << PAGE_CACHE_SHIFT;
> +		release_bytes = ALIGN(copied + sector_offset, root->sectorsize);
>
>   		if (copied > 0)
>   			ret = btrfs_dirty_pages(root, inode, pages,
> @@ -1632,8 +1649,7 @@ again:
>
>   		if (only_release_metadata && copied > 0) {
>   			lockstart = round_down(pos, root->sectorsize);
> -			lockend = lockstart +
> -				(dirty_pages << PAGE_CACHE_SHIFT) - 1;
> +			lockend = round_up(pos + copied, root->sectorsize) - 1;
>
>   			set_extent_bit(&BTRFS_I(inode)->io_tree, lockstart,
>   				       lockend, EXTENT_NORESERVE, NULL,
>