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

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Date:	Mon, 14 Dec 2015 19:33:49 -0600
From:	Abhi Das <adas@...hat.com>
To:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Cc:	swhiteho@...hat.com, Abhi Das <adas@...hat.com>
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 <adas@...hat.com>
---
 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

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