[Devel] [PATCH RHEL7 COMMIT] ms/mm: don't TestClearPageError in __filemap_fdatawait_range

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-862.vz7.48.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-862.el7
------>
commit 3d8e37fbc10e34394d621c785b5db9495aefe2f3
Author: Vasily Averin <vvs at virtuozzo.com>
Date:   Sat Jun 9 13:29:34 2018 +0300

    ms/mm: don't TestClearPageError in __filemap_fdatawait_range
    
    mainline commit 5e8fcc1 ("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 to wait for writeback
    to complete 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 at redhat.com>
    Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
---
 mm/filemap.c | 20 +++++---------------
 1 file changed, 5 insertions(+), 15 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 09467f821631..a324c9267277 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -391,17 +391,16 @@ 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_CACHE_SHIFT;
 	pgoff_t end = end_byte >> PAGE_CACHE_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) &&
@@ -418,14 +417,11 @@ 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;
 }
 
 /**
@@ -445,14 +441,8 @@ 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;
-
-	ret = __filemap_fdatawait_range(mapping, start_byte, end_byte);
-	ret2 = filemap_check_errors(mapping);
-	if (!ret)
-		ret = ret2;
-
-	return ret;
+	__filemap_fdatawait_range(mapping, start_byte, end_byte);
+	return filemap_check_errors(mapping);
 }
 EXPORT_SYMBOL(filemap_fdatawait_range);