linux-kernel - Re: [RFC v2 PATCH] fs: __generic_file_splice_read retry lookup on AOP_TRUNCATED

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Message-ID: <20151216153404.GC16918@quack.suse.cz>
Date:	Wed, 16 Dec 2015 16:34:04 +0100
From:	Jan Kara <jack@...e.cz>
To:	Abhi Das <adas@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Bob Peterson <rpeterso@...hat.com>
Subject: Re: [RFC v2 PATCH] fs: __generic_file_splice_read retry lookup on
 AOP_TRUNCATED_PAGE

On Tue 15-12-15 09:43:25, Abhi Das wrote:
> During testing, I discovered that __generic_file_splice_read() returns
> 0 (EOF) when aops->readpage fails with AOP_TRUNCATED_PAGE on the first
> page of a single/multi-page splice read operation. This EOF return code
> causes the userspace test to (correctly) report a zero-length read error
> when it was expecting otherwise.
> 
> The current strategy of returning a partial non-zero read when ->readpage
> returns AOP_TRUNCATED_PAGE works only when the failed page is not the
> first of the lot being processed.
> 
> This patch attempts to retry lookup and call ->readpage again on pages
> that had previously failed with AOP_TRUNCATED_PAGE. With this patch, my
> tests pass and I haven't noticed any unwanted side effects.
> 
> This version fixes a return code issue pointed out by Bob Peterson.
> 
> Signed-off-by: Abhi Das <adas@...hat.com>
> Cc: Bob Peterson <rpeterso@...hat.com>
> ---
>  fs/splice.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/splice.c b/fs/splice.c
> index 801c21c..365cd2a 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -387,6 +387,7 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
>  	spd.nr_pages = 0;
>  	for (page_nr = 0; page_nr < nr_pages; page_nr++) {
>  		unsigned int this_len;
> +		int retries = 0;
>  
>  		if (!len)
>  			break;
> @@ -415,6 +416,7 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
>  			 */
>  			if (!page->mapping) {
>  				unlock_page(page);
> +retry_lookup:
>  				page = find_or_create_page(mapping, index,
>  						mapping_gfp_mask(mapping));
>  
> @@ -439,14 +441,13 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
>  			error = mapping->a_ops->readpage(in, page);
>  			if (unlikely(error)) {
>  				/*
> -				 * We really should re-lookup the page here,
> -				 * but it complicates things a lot. Instead
> -				 * lets just do what we already stored, and
> -				 * we'll get it the next time we are called.
> +				 * Re-lookup the page
>  				 */
> -				if (error == AOP_TRUNCATED_PAGE)
> +				if (error == AOP_TRUNCATED_PAGE) {
>  					error = 0;
> -
> +					if (retries++ < 3)
> +						goto retry_lookup;
> +				}

I don't like this retry-three-times loop. That is still leaving the
possibility of 0 return just much less likely (so it will lead to even
weirder and harded to debug failures). IMO we should just terminate the
loop like we did previously if spd.nr_pages > 0 and we retry indefinitely
if it is the first page that failed to read.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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