2015-12-15 01:34:10

by Abhi Das

Subject: [RFC PATCH] fs: __generic_file_splice_read retry lookup on AOP_TRUNCATED_PAGE

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.

Signed-off-by: Abhi Das <[email protected]>
---
fs/splice.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/splice.c b/fs/splice.c
index 801c21c..c424d44 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));

@@ -439,13 +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) {
+ if (retries++ < 3)
+ goto retry_lookup;
error = 0;
+ }

break;
}
--
2.4.3

2015-12-15 13:45:25

by Bob Peterson

Subject: Re: [RFC PATCH] fs: __generic_file_splice_read retry lookup on AOP_TRUNCATED_PAGE

Hi Abhi,

Comment below.

----- Original Message -----
> 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.
>
> Signed-off-by: Abhi Das <[email protected]>
> ---
> fs/splice.c | 12 +++++++-----
> 1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/fs/splice.c b/fs/splice.c
> index 801c21c..c424d44 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));
>
> @@ -439,13 +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) {
> + if (retries++ < 3)
> + goto retry_lookup;
> error = 0;

I have no problem with doing the retry. However, I think we should probably set
error = 0 before we decide whether to retry here. Otherwise, if error is
AOP_TRUNCATED_PAGE, it might jump to retry_lookup, but then discover that the
page is PageUptodate(), goto fill_it with error still set to AOP_TRUNCATED_PAGE,
then end up returning that to the caller.

Regards,

Bob Peterson
Red Hat File Systems