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Date:   Mon, 9 Jan 2023 18:35:13 +0100
From:   Jan Kara <jack@suse.cz>
To:     David Howells <dhowells@redhat.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>, Jens Axboe <axboe@kernel.dk>,
        Christoph Hellwig <hch@lst.de>,
        Matthew Wilcox <willy@infradead.org>,
        Logan Gunthorpe <logang@deltatee.com>,
        Christoph Hellwig <hch@infradead.org>,
        Jeff Layton <jlayton@kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 7/7] iov_iter, block: Make bio structs pin pages
 rather than ref'ing if appropriate
Message-ID: <20230109173513.htfqbkrtqm52pnye@quack3>
References: <167305160937.1521586.133299343565358971.stgit@warthog.procyon.org.uk>
 <167305166150.1521586.10220949115402059720.stgit@warthog.procyon.org.uk>
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On Sat 07-01-23 00:34:21, David Howells wrote:
> Convert the block layer's bio code to use iov_iter_extract_pages() instead
> of iov_iter_get_pages().  This will pin pages or leave them unaltered
> rather than getting a ref on them as appropriate to the source iterator.
> 
> A field, bi_cleanup_mode, is added to the bio struct that gets set by
> iov_iter_extract_pages() with FOLL_* flags indicating what cleanup is
> necessary.  FOLL_GET -> put_page(), FOLL_PIN -> unpin_user_page().  Other
> flags could also be used in future.
> 
> Newly allocated bio structs have bi_cleanup_mode set to FOLL_GET to
> indicate that attached pages are ref'd by default.  Cloning sets it to 0.
> __bio_iov_iter_get_pages() overrides it to what iov_iter_extract_pages()
> indicates.
> 
> [!] Note that this is tested a bit with ext4, but nothing else.
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Al Viro <viro@zeniv.linux.org.uk>
> cc: Jens Axboe <axboe@kernel.dk>
> cc: Christoph Hellwig <hch@lst.de>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: Logan Gunthorpe <logang@deltatee.com>

So currently we already have BIO_NO_PAGE_REF flag and what you do in this
patch partially duplicates that. So either I'd drop that flag or instead of
bi_cleanup_mode variable (which honestly looks a bit wasteful given how we
microoptimize struct bio) just add another BIO_ flag...

								Honza

> ---
> 
>  block/bio.c               |   47 +++++++++++++++++++++++++++++++++------------
>  include/linux/blk_types.h |    1 +
>  2 files changed, 35 insertions(+), 13 deletions(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 5f96fcae3f75..eafcbeba0bab 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 bi_cleanup_mode to FOLL_GET, but this
> + * should be set to FOLL_PIN if the page should be unpinned instead; if the
> + * pages should not be put or unpinned, this should be set to 0
>   */
>  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->bi_cleanup_mode = FOLL_GET;
>  	bio->bi_vcnt = 0;
>  
>  	atomic_set(&bio->__bi_remaining, 1);
> @@ -302,6 +308,7 @@ void bio_reset(struct bio *bio, struct block_device *bdev, blk_opf_t opf)
>  {
>  	bio_uninit(bio);
>  	memset(bio, 0, BIO_RESET_BYTES);
> +	bio->bi_cleanup_mode = FOLL_GET;
>  	atomic_set(&bio->__bi_remaining, 1);
>  	bio->bi_bdev = bdev;
>  	if (bio->bi_bdev)
> @@ -814,6 +821,7 @@ 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->bi_cleanup_mode = 0;
>  
>  	if (bio->bi_bdev) {
>  		if (bio->bi_bdev == bio_src->bi_bdev &&
> @@ -1168,6 +1176,18 @@ bool bio_add_folio(struct bio *bio, struct folio *folio, size_t len,
>  	return bio_add_page(bio, &folio->page, len, off) > 0;
>  }
>  
> +/*
> + * Clean up a page according to the mode indicated by iov_iter_extract_pages(),
> + * where the page is may be pinned or may have a ref taken on it.
> + */
> +static void bio_release_page(struct bio *bio, struct page *page)
> +{
> +	if (bio->bi_cleanup_mode & FOLL_PIN)
> +		unpin_user_page(page);
> +	if (bio->bi_cleanup_mode & FOLL_GET)
> +		put_page(page);
> +}
> +
>  void __bio_release_pages(struct bio *bio, bool mark_dirty)
>  {
>  	struct bvec_iter_all iter_all;
> @@ -1176,7 +1196,7 @@ void __bio_release_pages(struct bio *bio, bool mark_dirty)
>  	bio_for_each_segment_all(bvec, bio, iter_all) {
>  		if (mark_dirty && !PageCompound(bvec->bv_page))
>  			set_page_dirty_lock(bvec->bv_page);
> -		put_page(bvec->bv_page);
> +		bio_release_page(bio, bvec->bv_page);
>  	}
>  }
>  EXPORT_SYMBOL_GPL(__bio_release_pages);
> @@ -1213,7 +1233,7 @@ static int bio_iov_add_page(struct bio *bio, struct page *page,
>  	}
>  
>  	if (same_page)
> -		put_page(page);
> +		bio_release_page(bio, page);
>  	return 0;
>  }
>  
> @@ -1227,7 +1247,7 @@ static int bio_iov_add_zone_append_page(struct bio *bio, struct page *page,
>  			queue_max_zone_append_sectors(q), &same_page) != len)
>  		return -EINVAL;
>  	if (same_page)
> -		put_page(page);
> +		bio_release_page(bio, page);
>  	return 0;
>  }
>  
> @@ -1238,10 +1258,10 @@ static int bio_iov_add_zone_append_page(struct bio *bio, struct page *page,
>   * @bio: bio to add pages to
>   * @iter: iov iterator describing the region to be mapped
>   *
> - * Pins pages from *iter and appends them to @bio's bvec array. The
> - * pages will have to be released using put_page() when done.
> - * For multi-segment *iter, this function only adds pages from the
> - * next non-empty segment of the iov iterator.
> + * Pins pages from *iter and appends them to @bio's bvec array.  The pages will
> + * have to be released using put_page() or unpin_user_page() when done as
> + * according to bi_cleanup_mode.  For multi-segment *iter, this function only
> + * adds pages from the next non-empty segment of the iov iterator.
>   */
>  static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>  {
> @@ -1273,9 +1293,10 @@ 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, &bio->bi_cleanup_mode);
>  	if (unlikely(size <= 0))
>  		return size ? size : -EFAULT;
>  
> @@ -1308,7 +1329,7 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>  	iov_iter_revert(iter, left);
>  out:
>  	while (i < nr_pages)
> -		put_page(pages[i++]);
> +		bio_release_page(bio, pages[i++]);
>  
>  	return ret;
>  }
> @@ -1489,8 +1510,8 @@ void bio_set_pages_dirty(struct bio *bio)
>   * the BIO and re-dirty the pages in process context.
>   *
>   * It is expected that bio_check_pages_dirty() will wholly own the BIO from
> - * here on.  It will run one put_page() against each page and will run one
> - * bio_put() against the BIO.
> + * here on.  It will run one put_page() or unpin_user_page() against each page
> + * and will run one bio_put() against the BIO.
>   */
>  
>  static void bio_dirty_fn(struct work_struct *work);
> diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
> index 99be590f952f..883f873a01ef 100644
> --- a/include/linux/blk_types.h
> +++ b/include/linux/blk_types.h
> @@ -289,6 +289,7 @@ struct bio {
>  #endif
>  	};
>  
> +	unsigned int		bi_cleanup_mode; /* How to clean up pages */
>  	unsigned short		bi_vcnt;	/* how many bio_vec's */
>  
>  	/*
> 
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR