linux-kernel - Re: [PATCH v4 7/7] iov_iter, block: Make bio structs pin pages rather than ref'ing if appropriate

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Date:   Mon, 9 Jan 2023 15:57:08 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     David Howells <dhowells@...hat.com>
Cc:     Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        Christoph Hellwig <hch@...radead.org>,
        Jeff Layton <jlayton@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 7/7] iov_iter, block: Make bio structs pin pages rather
 than ref'ing if appropriate

On 1/9/23 3:24?PM, David Howells wrote:
> Would you be okay with me flipping the logic of BIO_NO_PAGE_REF, so I end up
> with:
> 
> 	static void bio_release_page(struct bio *bio, struct page *page)
> 	{
> 		if (bio_flagged(bio, BIO_PAGE_PINNED))
> 			unpin_user_page(page);
> 		if (bio_flagged(bio, BIO_PAGE_REFFED))
> 			put_page(page);
> 	}
> 
> See attached patch.

I think it makes more sense to have NO_REF check, to be honest, as that
means the general path doesn't have to set that flag. But I don't feel
too strongly about that part.

> diff --git a/block/bio.c b/block/bio.c
> index 5f96fcae3f75..5b9f9fc62345 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -243,6 +243,11 @@ static void bio_free(struct bio *bio)
>   * Users of this function have their own bio allocation. Subsequently,
>   * they must remember to pair any call to bio_init() with bio_uninit()
>   * when IO has completed, or when the bio is released.
> + *
> + * We set the initial assumption that pages attached to the bio will be
> + * released with put_page() by setting BIO_PAGE_REFFED, but this should be set
> + * to BIO_PAGE_PINNED if the page should be unpinned instead; if the pages
> + * should not be put or unpinned, these flags should be cleared.
>   */
>  void bio_init(struct bio *bio, struct block_device *bdev, struct bio_vec *table,
>  	      unsigned short max_vecs, blk_opf_t opf)
> @@ -274,6 +279,7 @@ void bio_init(struct bio *bio, struct block_device *bdev, struct bio_vec *table,
>  #ifdef CONFIG_BLK_DEV_INTEGRITY
>  	bio->bi_integrity = NULL;
>  #endif
> +	bio_set_flag(bio, BIO_PAGE_REFFED);

This is first set to zero, then you set the flag. Why not just
initialize it like that to begin with?

> @@ -302,6 +308,8 @@ void bio_reset(struct bio *bio, struct block_device *bdev, blk_opf_t opf)
>  {
>  	bio_uninit(bio);
>  	memset(bio, 0, BIO_RESET_BYTES);
> +	bio_set_flag(bio, BIO_PAGE_REFFED);
> +	bio_clear_flag(bio, BIO_PAGE_PINNED);
>  	atomic_set(&bio->__bi_remaining, 1);
>  	bio->bi_bdev = bdev;
>  	if (bio->bi_bdev)

You just memset bi_flags here, surely we don't need to clear
BIO_PAGE_PINNED after that?

> @@ -814,6 +822,8 @@ static int __bio_clone(struct bio *bio, struct bio *bio_src, gfp_t gfp)
>  	bio_set_flag(bio, BIO_CLONED);
>  	bio->bi_ioprio = bio_src->bi_ioprio;
>  	bio->bi_iter = bio_src->bi_iter;
> +	bio_clear_flag(bio, BIO_PAGE_REFFED);
> +	bio_clear_flag(bio, BIO_PAGE_PINNED);

Maybe it would make sense to have a set/clear mask operation? Not
strictly required for this patch, but probably worth checking after the
fact.

> @@ -1273,12 +1295,20 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>  	 * result to ensure the bio's total size is correct. The remainder of
>  	 * the iov data will be picked up in the next bio iteration.
>  	 */
> -	size = iov_iter_get_pages(iter, pages,
> -				  UINT_MAX - bio->bi_iter.bi_size,
> -				  nr_pages, &offset, gup_flags);
> +	size = iov_iter_extract_pages(iter, &pages,
> +				      UINT_MAX - bio->bi_iter.bi_size,
> +				      nr_pages, gup_flags,
> +				      &offset, &cleanup_mode);
>  	if (unlikely(size <= 0))
>  		return size ? size : -EFAULT;
>  
> +	bio_clear_flag(bio, BIO_PAGE_REFFED);
> +	bio_clear_flag(bio, BIO_PAGE_PINNED);
> +	if (cleanup_mode & FOLL_GET)
> +		bio_set_flag(bio, BIO_PAGE_REFFED);
> +	if (cleanup_mode & FOLL_PIN)
> +		bio_set_flag(bio, BIO_PAGE_PINNED);
> +
>  	nr_pages = DIV_ROUND_UP(offset + size, PAGE_SIZE);

The cleanup_mode pass-back isn't the prettiest thing in the world, and
that's a lot of arguments. Maybe it'd be slightly better if we just have
gup_flags be an output parameter too?

Also not great to first clear both flags, then set them with the two
added branches...

> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 22078a28d7cb..1c6f051f6ff2 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -482,7 +482,8 @@ void zero_fill_bio(struct bio *bio);
>  
>  static inline void bio_release_pages(struct bio *bio, bool mark_dirty)
>  {
> -	if (!bio_flagged(bio, BIO_NO_PAGE_REF))
> +	if (bio_flagged(bio, BIO_PAGE_REFFED) ||
> +	    bio_flagged(bio, BIO_PAGE_PINNED))
>  		__bio_release_pages(bio, mark_dirty);
>  }

Same here on a mask check, but perhaps it ends up generating the same
code?

-- 
Jens Axboe