Re: [PATCH] btrfs: Refactor main loop in extent_readpages

[David Sterba <dsterba@suse.cz>]

On Thu, Nov 29, 2018 at 06:41:31PM +0200, Nikolay Borisov wrote:
> extent_readpages processes all pages in the readlist in batches of 16,
> this is implemented by a single for loop but thanks to an if condition
> the loop does 2 things based on whether we've filled the batch or not.
> Additionally due to the structure of the code there is an additional
> check which deals with partial batches.
> 
> Streamline all of this by explicitly using two loops. The outter one is
> used to process all pages while the inner one just fills in the batch
> of 16 (currently). Due to this new structure the code guarantees that
> all pages are processed in the loop hence the code to deal with any
> leftovers is eliminated.
> 
> This also enable the compiler to inline __extent_readpages:
> 
> 	./scripts/bloat-o-meter fs/btrfs/extent_io.o extent_io.for
> 
> 	add/remove: 0/1 grow/shrink: 1/0 up/down: 660/-820 (-160)
> 	Function                                     old     new   delta
> 	extent_readpages                             476    1136    +660
> 	__extent_readpages                           820       -    -820
> 	Total: Before=44315, After=44155, chg -0.36%
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  fs/btrfs/extent_io.c | 37 ++++++++++++++++---------------------
>  1 file changed, 16 insertions(+), 21 deletions(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 8332c5f4b1c3..233f835dd6f8 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -4093,43 +4093,38 @@ int extent_writepages(struct address_space *mapping,
>  int extent_readpages(struct address_space *mapping, struct list_head *pages,
>  		     unsigned nr_pages)

nr_pages becomes unused now. I've checked the MM code that calls here
and it's guaranteed that the number of list 'pages' members is same as
nr_pages.

>  {
> +#define BTRFS_PAGES_BATCH 16
> +
>  	struct bio *bio = NULL;
> -	unsigned page_idx;
>  	unsigned long bio_flags = 0;
> -	struct page *pagepool[16];
> -	struct page *page;
> +	struct page *pagepool[BTRFS_PAGES_BATCH];

I don't think we need the extra define, if

	struct page *pagepool[16];

	...

	for (nr = 0; nr < ARRAY_SIZE(pagepool) ...

Otherwise ok, nice cleanup.

Reviewed-by: David Sterba <dsterba@suse.com>