Re: [PATCH v6 17/19] iomap: Restructure iomap_readpages_actor

From: John Hubbard <jhubbard@nvidia.com>
To: Matthew Wilcox <willy@infradead.org>, <linux-fsdevel@vger.kernel.org>
Cc: <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
	<linux-btrfs@vger.kernel.org>, <linux-erofs@lists.ozlabs.org>,
	<linux-ext4@vger.kernel.org>,
	<linux-f2fs-devel@lists.sourceforge.net>,
	<cluster-devel@redhat.com>, <ocfs2-devel@oss.oracle.com>,
	<linux-xfs@vger.kernel.org>
Subject: Re: [PATCH v6 17/19] iomap: Restructure iomap_readpages_actor
Date: Tue, 18 Feb 2020 19:17:18 -0800	[thread overview]
Message-ID: <d4803ef9-7a2f-965f-8f0f-c5e15396d892@nvidia.com> (raw)
In-Reply-To: <20200217184613.19668-31-willy@infradead.org>

On 2/17/20 10:46 AM, 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.

Also added a new WARN_ON() and BUG(), although I'm wondering about the BUG
below...

> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.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) {

nit: this "for" loop was perfect just the way it was. :) I'd vote here for reverting
the change to a "while" loop. Because with this change, now the code has to 
separately initialize "done", separately increment "done", and the beauty of a
for loop is that the loop init and control is all clearly in one place. For things
that follow that model (as in this case!), that's a Good Thing.

And I don't see any technical reason (even in the following patch) that requires 
this change.

> -		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;
> +		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);

Is a full BUG_ON() definitely called for here? Seems like a WARN might suffice...

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

thanks,
-- 
John Hubbard
NVIDIA