Re: [PATCH v2 6/9] iomap: Convert read_count to read_bytes_pending

[Darrick J. Wong]

On Fri, Sep 11, 2020 at 12:47:04AM +0100, Matthew Wilcox (Oracle) wrote:
> Instead of counting bio segments, count the number of bytes submitted.
> This insulates us from the block layer's definition of what a 'same page'
> is, which is not necessarily clear once THPs are involved.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
>  fs/iomap/buffered-io.c | 41 ++++++++++++-----------------------------
>  1 file changed, 12 insertions(+), 29 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 9670c096b83e..1cf976a8e55c 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -26,7 +26,7 @@
>   * to track sub-page uptodate status and I/O completions.
>   */
>  struct iomap_page {
> -	atomic_t		read_count;
> +	atomic_t		read_bytes_pending;
>  	atomic_t		write_count;
>  	spinlock_t		uptodate_lock;
>  	unsigned long		uptodate[];
> @@ -72,7 +72,7 @@ iomap_page_release(struct page *page)
>  
>  	if (!iop)
>  		return;
> -	WARN_ON_ONCE(atomic_read(&iop->read_count));
> +	WARN_ON_ONCE(atomic_read(&iop->read_bytes_pending));
>  	WARN_ON_ONCE(atomic_read(&iop->write_count));
>  	WARN_ON_ONCE(bitmap_full(iop->uptodate, nr_blocks) !=
>  			PageUptodate(page));
> @@ -167,13 +167,6 @@ iomap_set_range_uptodate(struct page *page, unsigned off, unsigned len)
>  		SetPageUptodate(page);
>  }
>  
> -static void
> -iomap_read_finish(struct iomap_page *iop, struct page *page)
> -{
> -	if (!iop || atomic_dec_and_test(&iop->read_count))
> -		unlock_page(page);
> -}
> -
>  static void
>  iomap_read_page_end_io(struct bio_vec *bvec, int error)
>  {
> @@ -187,7 +180,8 @@ iomap_read_page_end_io(struct bio_vec *bvec, int error)
>  		iomap_set_range_uptodate(page, bvec->bv_offset, bvec->bv_len);
>  	}
>  
> -	iomap_read_finish(iop, page);
> +	if (!iop || atomic_sub_and_test(bvec->bv_len, &iop->read_bytes_pending))
> +		unlock_page(page);
>  }
>  
>  static void
> @@ -267,30 +261,19 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>  	}
>  
>  	ctx->cur_page_in_bio = true;
> +	if (iop)
> +		atomic_add(plen, &iop->read_bytes_pending);
>  
> -	/*
> -	 * Try to merge into a previous segment if we can.
> -	 */
> +	/* Try to merge into a previous segment if we can */
>  	sector = iomap_sector(iomap, pos);
> -	if (ctx->bio && bio_end_sector(ctx->bio) == sector)
> +	if (ctx->bio && bio_end_sector(ctx->bio) == sector) {
> +		if (__bio_try_merge_page(ctx->bio, page, plen, poff,
> +				&same_page))
> +			goto done;
>  		is_contig = true;
> -
> -	if (is_contig &&
> -	    __bio_try_merge_page(ctx->bio, page, plen, poff, &same_page)) {
> -		if (!same_page && iop)
> -			atomic_inc(&iop->read_count);
> -		goto done;
>  	}
>  
> -	/*
> -	 * If we start a new segment we need to increase the read count, and we
> -	 * need to do so before submitting any previous full bio to make sure
> -	 * that we don't prematurely unlock the page.
> -	 */
> -	if (iop)
> -		atomic_inc(&iop->read_count);
> -
> -	if (!ctx->bio || !is_contig || bio_full(ctx->bio, plen)) {
> +	if (!is_contig || bio_full(ctx->bio, plen)) {
>  		gfp_t gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL);
>  		gfp_t orig_gfp = gfp;
>  		int nr_vecs = (length + PAGE_SIZE - 1) >> PAGE_SHIFT;
> -- 
> 2.28.0
>