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Date: Fri, 21 Feb 2020 16:44:25 -0800
From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v7 21/24] iomap: Restructure iomap_readpages_actor
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On Wed, Feb 19, 2020 at 01:01:00PM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.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@infradead.org>
> ---
>  fs/iomap/buffered-io.c | 32 ++++++++++++++++++--------------
>  1 file changed, 18 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index cb3511eb152a..31899e6cb0f8 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;
> -
> -	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;
> -		}
> +	loff_t ret, done = 0;
> +
> +	while (done < length) {
>  		if (!ctx->cur_page) {
>  			ctx->cur_page = iomap_next_page(inode, ctx->pages,
>  					pos, length, &done);
> @@ -418,6 +412,20 @@ iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
>  		}
>  		ret = iomap_readpage_actor(inode, pos + done, length - done,
>  				ctx, iomap, srcmap);
> +		done += ret;
> +
> +		/* Keep working on a partial page */
> +		if (ret && offset_in_page(pos + done))
> +			continue;
> +
> +		if (!ctx->cur_page_in_bio)
> +			unlock_page(ctx->cur_page);
> +		put_page(ctx->cur_page);
> +		ctx->cur_page = NULL;
> +
> +		/* Don't loop forever if we made no progress */
> +		if (WARN_ON(!ret))
> +			break;
>  	}
>  
>  	return done;
> @@ -451,11 +459,7 @@ 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);

Whoah, is the system totally unrecoverably hosed at this point?

I get that this /shouldn't/ happen, but should we somehow end up with a
page here, are we unable either to release it or even just leak it?  I'd
have thought a WARN_ON would be just fine here.

--D

>  
>  	/*
>  	 * Check that we didn't lose a page due to the arcance calling
> -- 
> 2.25.0
>