Re: [PATCH v4 08/18] btrfs: introduce helper for subpage uptodate status

[Josef Bacik <josef@toxicpanda.com>]

On 1/16/21 2:15 AM, Qu Wenruo wrote:
> This patch introduce the following functions to handle btrfs subpage
> uptodate status:
> - btrfs_subpage_set_uptodate()
> - btrfs_subpage_clear_uptodate()
> - btrfs_subpage_test_uptodate()
>    Those helpers can only be called when the range is ensured to be
>    inside the page.
> 
> - btrfs_page_set_uptodate()
> - btrfs_page_clear_uptodate()
> - btrfs_page_test_uptodate()
>    Those helpers can handle both regular sector size and subpage without
>    problem.
>    Although caller should still ensure that the range is inside the page.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>   fs/btrfs/subpage.h | 115 +++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 115 insertions(+)
> 
> diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
> index d8b34879368d..3373ef4ffec1 100644
> --- a/fs/btrfs/subpage.h
> +++ b/fs/btrfs/subpage.h
> @@ -23,6 +23,7 @@
>   struct btrfs_subpage {
>   	/* Common members for both data and metadata pages */
>   	spinlock_t lock;
> +	u16 uptodate_bitmap;
>   	union {
>   		/* Structures only used by metadata */
>   		bool under_alloc;
> @@ -78,4 +79,118 @@ static inline void btrfs_page_end_meta_alloc(struct btrfs_fs_info *fs_info,
>   int btrfs_attach_subpage(struct btrfs_fs_info *fs_info, struct page *page);
>   void btrfs_detach_subpage(struct btrfs_fs_info *fs_info, struct page *page);
>   
> +/*
> + * Convert the [start, start + len) range into a u16 bitmap
> + *
> + * E.g. if start == page_offset() + 16K, len = 16K, we get 0x00f0.
> + */
> +static inline u16 btrfs_subpage_calc_bitmap(struct btrfs_fs_info *fs_info,
> +			struct page *page, u64 start, u32 len)
> +{
> +	int bit_start = offset_in_page(start) >> fs_info->sectorsize_bits;
> +	int nbits = len >> fs_info->sectorsize_bits;
> +
> +	/* Basic checks */
> +	ASSERT(PagePrivate(page) && page->private);
> +	ASSERT(IS_ALIGNED(start, fs_info->sectorsize) &&
> +	       IS_ALIGNED(len, fs_info->sectorsize));
> +
> +	/*
> +	 * The range check only works for mapped page, we can
> +	 * still have unampped page like dummy extent buffer pages.
> +	 */
> +	if (page->mapping)
> +		ASSERT(page_offset(page) <= start &&
> +			start + len <= page_offset(page) + PAGE_SIZE);

Once you gate the helpers on UNMAPPED you'll always have page->mapping set and 
you can drop the if statement.  Thanks,

Josef