linux-kernel - [PATCH v2 13/17] mm: don't TestClearPageError in __filemap_fdatawait

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Message-Id: <20170412120614.6111-14-jlayton@redhat.com>
Date:   Wed, 12 Apr 2017 08:06:10 -0400
From:   Jeff Layton <jlayton@...hat.com>
To:     linux-fsdevel@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        tytso@....edu, jack@...e.cz, willy@...radead.org, neilb@...e.com,
        viro@...iv.linux.org.uk
Subject: [PATCH v2 13/17] mm: don't TestClearPageError in __filemap_fdatawait_range

The -EIO returned here can end up overriding whatever error is marked in
the address space, and be returned at fsync time, even when there is a
more appropriate error stored in the mapping.

Read errors are also sometimes tracked on a per-page level using
PG_error. Suppose we have a read error on a page, and then that page is
subsequently dirtied by overwriting the whole page. Writeback doesn't
clear PG_error, so we can then end up successfully writing back that
page and still return -EIO on fsync.

Worse yet, PG_error is cleared during a sync() syscall, but the -EIO
return from that is silently discarded. Any subsystem that is relying on
PG_error to report errors during fsync can easily lose writeback errors
due to this. All you need is a stray sync() call on the box at the wrong
time and you've lost the error.

Since the handling of the PG_error flag is somewhat inconsistent across
subsystems, let's just rely on marking the address space when there are
writeback errors. Change the TestClearPageError call to ClearPageError,
and make __filemap_fdatawait_range a void return function.

Signed-off-by: Jeff Layton <jlayton@...hat.com>
---
 mm/filemap.c | 19 +++++--------------
 1 file changed, 5 insertions(+), 14 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 525dddc15abb..b43975ca7a2e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -362,17 +362,16 @@ int filemap_flush(struct address_space *mapping)
 }
 EXPORT_SYMBOL(filemap_flush);

-static int __filemap_fdatawait_range(struct address_space *mapping,
+static void __filemap_fdatawait_range(struct address_space *mapping,
 				     loff_t start_byte, loff_t end_byte)
 {
 	pgoff_t index = start_byte >> PAGE_SHIFT;
 	pgoff_t end = end_byte >> PAGE_SHIFT;
 	struct pagevec pvec;
 	int nr_pages;
-	int ret = 0;

 	if (end_byte < start_byte)
-		goto out;
+		return;

 	pagevec_init(&pvec, 0);
 	while ((index <= end) &&
@@ -389,14 +388,11 @@ static int __filemap_fdatawait_range(struct address_space *mapping,
 				continue;

 			wait_on_page_writeback(page);
-			if (TestClearPageError(page))
-				ret = -EIO;
+			ClearPageError(page);
 		}
 		pagevec_release(&pvec);
 		cond_resched();
 	}
-out:
-	return ret;
 }

 /**
@@ -416,15 +412,10 @@ static int __filemap_fdatawait_range(struct address_space *mapping,
 int filemap_fdatawait_range(struct address_space *mapping, loff_t start_byte,
 			    loff_t end_byte)
 {
-	int ret, ret2;
 	wb_err_t since = READ_ONCE(mapping->wb_err);

-	ret = __filemap_fdatawait_range(mapping, start_byte, end_byte);
-	ret2 = filemap_check_wb_error(mapping, since);
-	if (!ret)
-		ret = ret2;
-
-	return ret;
+	__filemap_fdatawait_range(mapping, start_byte, end_byte);
+	return filemap_check_wb_error(mapping, since);
 }
 EXPORT_SYMBOL(filemap_fdatawait_range);

-- 
2.9.3