linux-kernel - Re: [PATCH] fix unbalanced page refcounting in bio_map_user

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Date:   Sat, 23 Sep 2017 17:39:28 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Vitaly Mayatskikh <v.mayatskih@...il.com>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] fix unbalanced page refcounting in bio_map_user_iov

On Fri, Sep 22, 2017 at 01:18:39AM -0400, Vitaly Mayatskikh wrote:
> bio_map_user_iov and bio_unmap_user do unbalanced pages refcounting if
> IO vector has small consecutive buffers belonging to the same page.
> bio_add_pc_page merges them into one, but the page reference is never
> dropped.
> 
> Signed-off-by: Vitaly Mayatskikh <v.mayatskih@...il.com>
> 
> diff --git a/block/bio.c b/block/bio.c
> index b38e962fa83e..10cd3b6bed27 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1383,6 +1383,7 @@ struct bio *bio_map_user_iov(struct request_queue *q,
>  		offset = offset_in_page(uaddr);
>  		for (j = cur_page; j < page_limit; j++) {
>  			unsigned int bytes = PAGE_SIZE - offset;
> +			unsigned short prev_bi_vcnt = bio->bi_vcnt;
>  
>  			if (len <= 0)
>  				break;
> @@ -1397,6 +1398,13 @@ struct bio *bio_map_user_iov(struct request_queue *q,
>  					    bytes)
>  				break;
>  
> +			/*
> +			 * check if vector was merged with previous
> +			 * drop page reference if needed
> +			 */
> +			if (bio->bi_vcnt == prev_bi_vcnt)
> +				put_page(pages[j]);
> +

Except that now you've got double-puts on failure exits ;-/