linux-kernel - Re: [PATCH] dm-writecache: simplify the using of pointer wb

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Date:   Tue, 23 Apr 2019 11:49:31 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Huaisheng Ye <huye@...hat.com>
cc:     snitzer@...hat.com, agk@...hat.com, prarit@...hat.com,
        chengnt@...ovo.com, dm-devel@...hat.com,
        linux-kernel@...r.kernel.org, Huaisheng Ye <yehs1@...ovo.com>
Subject: Re: [PATCH] dm-writecache: simplify the using of pointer wb



On Fri, 12 Apr 2019, Huaisheng Ye wrote:

> From: Huaisheng Ye <yehs1@...ovo.com>
> 
> In function __writecache_writeback_pmem, the pointer bio has been
> already there, it doesn't need to seek bio's address from pointer wb.

The compiler optimizer knows that the offset between "wb" and "bio" is 
constant, so that there is no difference in the generated code before or 
after this patch.

Mikulas

> Signed-off-by: Huaisheng Ye <yehs1@...ovo.com>
> ---
>  drivers/md/dm-writecache.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
> index f782287..5b4d1c1 100644
> --- a/drivers/md/dm-writecache.c
> +++ b/drivers/md/dm-writecache.c
> @@ -1478,9 +1478,9 @@ static void __writecache_writeback_pmem(struct dm_writecache *wc, struct writeba
>  		bio = bio_alloc_bioset(GFP_NOIO, max_pages, &wc->bio_set);
>  		wb = container_of(bio, struct writeback_struct, bio);
>  		wb->wc = wc;
> -		wb->bio.bi_end_io = writecache_writeback_endio;
> -		bio_set_dev(&wb->bio, wc->dev->bdev);
> -		wb->bio.bi_iter.bi_sector = read_original_sector(wc, e);
> +		bio->bi_end_io = writecache_writeback_endio;
> +		bio_set_dev(bio, wc->dev->bdev);
> +		bio->bi_iter.bi_sector = read_original_sector(wc, e);
>  		wb->page_offset = PAGE_SIZE;
>  		if (max_pages <= WB_LIST_INLINE ||
>  		    unlikely(!(wb->wc_list = kmalloc_array(max_pages, sizeof(struct wc_entry *),
> @@ -1507,12 +1507,12 @@ static void __writecache_writeback_pmem(struct dm_writecache *wc, struct writeba
>  			wb->wc_list[wb->wc_list_n++] = f;
>  			e = f;
>  		}
> -		bio_set_op_attrs(&wb->bio, REQ_OP_WRITE, WC_MODE_FUA(wc) * REQ_FUA);
> +		bio_set_op_attrs(bio, REQ_OP_WRITE, WC_MODE_FUA(wc) * REQ_FUA);
>  		if (writecache_has_error(wc)) {
>  			bio->bi_status = BLK_STS_IOERR;
> -			bio_endio(&wb->bio);
> +			bio_endio(bio);
>  		} else {
> -			submit_bio(&wb->bio);
> +			submit_bio(bio);
>  		}
>  
>  		__writeback_throttle(wc, wbl);
> -- 
> 1.8.3.1
>