linux-ext4 - Re: [PATCH v6 17/19] iomap: Restructure iomap_readpages

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Date:   Wed, 19 Feb 2020 14:29:00 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-erofs@...ts.ozlabs.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        ocfs2-devel@....oracle.com, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v6 17/19] iomap: Restructure iomap_readpages_actor

On Mon, Feb 17, 2020 at 10:46:11AM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> 
> By putting the 'have we reached the end of the page' condition at the end
> of the loop instead of the beginning, we can remove the 'submit the last
> page' code from iomap_readpages().  Also check that iomap_readpage_actor()
> didn't return 0, which would lead to an endless loop.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  fs/iomap/buffered-io.c | 25 ++++++++++++-------------
>  1 file changed, 12 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index cb3511eb152a..44303f370b2d 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -400,15 +400,9 @@ iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
>  		void *data, struct iomap *iomap, struct iomap *srcmap)
>  {
>  	struct iomap_readpage_ctx *ctx = data;
> -	loff_t done, ret;
> +	loff_t ret, done = 0;
>  
> -	for (done = 0; done < length; done += ret) {
> -		if (ctx->cur_page && offset_in_page(pos + done) == 0) {
> -			if (!ctx->cur_page_in_bio)
> -				unlock_page(ctx->cur_page);
> -			put_page(ctx->cur_page);
> -			ctx->cur_page = NULL;
> -		}
> +	while (done < length) {
>  		if (!ctx->cur_page) {
>  			ctx->cur_page = iomap_next_page(inode, ctx->pages,
>  					pos, length, &done);
> @@ -418,6 +412,15 @@ iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
>  		}
>  		ret = iomap_readpage_actor(inode, pos + done, length - done,
>  				ctx, iomap, srcmap);
> +		if (WARN_ON(ret == 0))
> +			break;

This error case now leaks ctx->cur_page....

> +		done += ret;
> +		if (offset_in_page(pos + done) == 0) {
> +			if (!ctx->cur_page_in_bio)
> +				unlock_page(ctx->cur_page);
> +			put_page(ctx->cur_page);
> +			ctx->cur_page = NULL;
> +		}
>  	}
>  
>  	return done;
> @@ -451,11 +454,7 @@ iomap_readpages(struct address_space *mapping, struct list_head *pages,
>  done:
>  	if (ctx.bio)
>  		submit_bio(ctx.bio);
> -	if (ctx.cur_page) {
> -		if (!ctx.cur_page_in_bio)
> -			unlock_page(ctx.cur_page);
> -		put_page(ctx.cur_page);
> -	}
> +	BUG_ON(ctx.cur_page);

And so will now trigger both a warn and a bug....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com