Re: [PATCH] bio: limit bio max size.

[Damien Le Moal]

On 2021/01/12 17:52, Changheun Lee wrote:
> From: "Changheun Lee" <nanich.lee@xxxxxxxxxxx>
> 
> bio size can grow up to 4GB when muli-page bvec is enabled.
> but sometimes it would lead to inefficient behaviors.
> in case of large chunk direct I/O, - 64MB chunk read in user space -
> all pages for 64MB would be merged to a bio structure if memory address is
> continued phsycally. it makes some delay to submit until merge complete.
> bio max size should be limited as a proper size.

But merging physically contiguous pages into the same bvec + later automatic bio
split on submit should give you better throughput for large IOs compared to
having to issue a bio chain of smaller BIOs that are arbitrarily sized and will
likely need splitting anyway (because of DMA boundaries etc).

Do you have a specific case where you see higher performance with this patch
applied ? On Intel, BIO_MAX_SIZE would be 1MB... That is arbitrary and too small
considering that many hardware can execute larger IOs than that.


> 
> Signed-off-by: Changheun Lee <nanich.lee@xxxxxxxxxxx>
> ---
>  block/bio.c         | 2 +-
>  include/linux/bio.h | 3 ++-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 1f2cc1fbe283..dbe14d675f28 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -877,7 +877,7 @@ bool __bio_try_merge_page(struct bio *bio, struct page *page,
>  		struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
>  
>  		if (page_is_mergeable(bv, page, len, off, same_page)) {
> -			if (bio->bi_iter.bi_size > UINT_MAX - len) {
> +			if (bio->bi_iter.bi_size > BIO_MAX_SIZE - len) {
>  				*same_page = false;
>  				return false;
>  			}
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 1edda614f7ce..0f49b354b1f6 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -20,6 +20,7 @@
>  #endif
>  
>  #define BIO_MAX_PAGES		256
> +#define BIO_MAX_SIZE		(BIO_MAX_PAGES * PAGE_SIZE)
>  
>  #define bio_prio(bio)			(bio)->bi_ioprio
>  #define bio_set_prio(bio, prio)		((bio)->bi_ioprio = prio)
> @@ -113,7 +114,7 @@ static inline bool bio_full(struct bio *bio, unsigned len)
>  	if (bio->bi_vcnt >= bio->bi_max_vecs)
>  		return true;
>  
> -	if (bio->bi_iter.bi_size > UINT_MAX - len)
> +	if (bio->bi_iter.bi_size > BIO_MAX_SIZE - len)
>  		return true;
>  
>  	return false;
> 


-- 
Damien Le Moal
Western Digital Research