[PATCH v2 8/9] mm: don't TestClearPageError in __filemap_fdatawait_range

[Jeff Layton]

The -EIO returned here can end up overriding whatever error is marked in
the address space. This means that an -ENOSPC error (AS_ENOSPC) can end
up being turned into -EIO if a page gets PG_error set on it during error
handling.

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 this code is silently discarded. Any subsystem that is
relying on PG_error to report errors during fsync or close is already
broken 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 
---
 mm/filemap.c | 20 +---
 1 file changed, 5 insertions(+), 15 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index fc123b9833e1..150559e94f8a 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -376,17 +376,16 @@ 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(, 0);
while ((index <= end) &&
@@ -403,14 +402,11 @@ static int __filemap_fdatawait_range(struct address_space 
*mapping,
continue;
 
wait_on_page_writeback(page);
-   if (TestClearPageError(page))
-   ret = -EIO;
+   ClearPageError(page);
}
pagevec_release();
cond_resched();
}
-out:
-   return ret;
 }
 
 /**
@@ -430,14 +426,8 @@ 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);
 
-- 
2.9.3

[PATCH v2 8/9] mm: don't TestClearPageError in __filemap_fdatawait_range

[Jeff Layton]

The -EIO returned here can end up overriding whatever error is marked in
the address space. This means that an -ENOSPC error (AS_ENOSPC) can end
up being turned into -EIO if a page gets PG_error set on it during error
handling.

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 this code is silently discarded. Any subsystem that is
relying on PG_error to report errors during fsync or close is already
broken 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 
---
 mm/filemap.c | 20 +---
 1 file changed, 5 insertions(+), 15 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index fc123b9833e1..150559e94f8a 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -376,17 +376,16 @@ 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(, 0);
while ((index <= end) &&
@@ -403,14 +402,11 @@ static int __filemap_fdatawait_range(struct address_space 
*mapping,
continue;
 
wait_on_page_writeback(page);
-   if (TestClearPageError(page))
-   ret = -EIO;
+   ClearPageError(page);
}
pagevec_release();
cond_resched();
}
-out:
-   return ret;
 }
 
 /**
@@ -430,14 +426,8 @@ 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);
 
-- 
2.9.3