@@ -576,6 +576,11 @@ should clear PG_Dirty and set PG_Writeback. It can be actually
written at any point after PG_Dirty is clear. Once it is known to be
safe, PG_Writeback is cleared.
+If there is an error during writeback, then the address_space should be
+marked with an error (typically using filemap_set_wb_error), in order to
+ensure that the error can later be reported to the application when an
+fsync is issued.
+
Writeback makes use of a writeback_control structure...
struct address_space_operations
@@ -888,7 +893,10 @@ otherwise noted.
release: called when the last reference to an open file is closed
- fsync: called by the fsync(2) system call
+ fsync: called by the fsync(2) system call. Filesystems that use the
+ pagecache should call filemap_report_wb_error before returning
+ to ensure that any errors that occurred during writeback are
+ reported and the file's error sequence advanced.
fasync: called by the fcntl(2) system call when asynchronous
(non-blocking) mode is enabled for a file
@@ -709,6 +709,9 @@ static int do_dentry_open(struct file *f,
f->f_inode = inode;
f->f_mapping = inode->i_mapping;
+ /* Ensure that we skip any errors that predate opening of the file */
+ f->f_wb_err = filemap_sample_wb_error(f->f_mapping);
+
if (unlikely(f->f_flags & O_PATH)) {
f->f_mode = FMODE_PATH;
f->f_op = &empty_fops;
@@ -31,6 +31,7 @@
#include <linux/workqueue.h>
#include <linux/percpu-rwsem.h>
#include <linux/delayed_call.h>
+#include <linux/errseq.h>
#include <asm/byteorder.h>
#include <uapi/linux/fs.h>
@@ -394,6 +395,7 @@ struct address_space {
gfp_t gfp_mask; /* implicit gfp mask for allocations */
struct list_head private_list; /* ditto */
void *private_data; /* ditto */
+ errseq_t wb_err;
} __attribute__((aligned(sizeof(long))));
/*
* On most architectures that alignment is already the case; but
@@ -846,6 +848,7 @@ struct file {
* Must not be taken from IRQ context.
*/
spinlock_t f_lock;
+ errseq_t f_wb_err;
atomic_long_t f_count;
unsigned int f_flags;
fmode_t f_mode;
@@ -2521,6 +2524,27 @@ extern int __filemap_fdatawrite_range(struct address_space *mapping,
extern int filemap_fdatawrite_range(struct address_space *mapping,
loff_t start, loff_t end);
extern int filemap_check_errors(struct address_space *mapping);
+extern int __must_check filemap_report_wb_error(struct file *file);
+
+/**
+ * filemap_check_wb_error - has an error occurred since the mark was sampled?
+ * @mapping: mapping to check for writeback errors
+ * @since: previously-sampled errseq_t
+ *
+ * Grab the errseq_t value from the mapping, and see if it has changed "since"
+ * the given value was sampled.
+ *
+ * If it has then report the latest error set, otherwise return 0.
+ */
+static inline int filemap_check_wb_error(struct address_space *mapping, errseq_t since)
+{
+ return errseq_check(&mapping->wb_err, since);
+}
+
+static inline errseq_t filemap_sample_wb_error(struct address_space *mapping)
+{
+ return errseq_sample(&mapping->wb_err);
+}
extern int vfs_fsync_range(struct file *file, loff_t start, loff_t end,
int datasync);
@@ -546,6 +546,44 @@ int filemap_write_and_wait_range(struct address_space *mapping,
EXPORT_SYMBOL(filemap_write_and_wait_range);
/**
+ * filemap_report_wb_error - report wb error (if any) that was previously set
+ * @file: struct file on which the error is being reported
+ *
+ * When userland calls fsync (or something like nfsd does the equivalent), we
+ * want to report any writeback errors that occurred since the last fsync (or
+ * since the file was opened if there haven't been any).
+ *
+ * Grab the wb_err from the mapping. If it matches what we have in the file,
+ * then just quickly return 0. The file is all caught up.
+ *
+ * If it doesn't match, then take the mapping value, set the "seen" flag in
+ * it and try to swap it into place. If it works, or another task beat us
+ * to it with the new value, then update the f_wb_err and return the error
+ * portion. The error at this point must be reported via proper channels
+ * (a'la fsync, or NFS COMMIT operation, etc.).
+ *
+ * While we handle mapping->wb_err with atomic operations, the f_wb_err
+ * value is protected by the f_lock since we must ensure that it reflects
+ * the latest value swapped in for this file descriptor.
+ */
+int filemap_report_wb_error(struct file *file)
+{
+ int err = 0;
+ struct address_space *mapping = file->f_mapping;
+
+ /* Locklessly handle the common case where nothing has changed */
+ if (errseq_check(&mapping->wb_err, READ_ONCE(file->f_wb_err))) {
+ /* Something changed, must use slow path */
+ spin_lock(&file->f_lock);
+ err = errseq_check_and_advance(&mapping->wb_err,
+ &file->f_wb_err);
+ spin_unlock(&file->f_lock);
+ }
+ return err;
+}
+EXPORT_SYMBOL(filemap_report_wb_error);
+
+/**
* replace_page_cache_page - replace a pagecache page with a new one
* @old: page to be replaced
* @new: page to replace with
Most filesystems currently use mapping_set_error and filemap_check_errors for setting and reporting/clearing writeback errors at the mapping level. filemap_check_errors is indirectly called from most of the filemap_fdatawait_* functions and from filemap_write_and_wait*. These functions are called from all sorts of contexts to wait on writeback to finish -- e.g. mostly in fsync, but also in truncate calls, getattr, etc. The non-fsync callers are problematic. We should be reporting writeback errors during fsync, but many places spread over the tree clear out errors before they can be properly reported, or report errors at nonsensical times. If I get -EIO on a stat() call, there is no reason for me to assume that it is because some previous writeback failed. The fact that it also clears out the error such that a subsequent fsync returns 0 is a bug, and a nasty one since that's potentially silent data corruption. This patch adds a small bit of new infrastructure for setting and reporting errors during address_space writeback. While the above was my original impetus for adding this, I think it's also the case that current fsync semantics are just problematic for userland. Most applications that call fsync do so to ensure that the data they wrote has hit the backing store. In the case where there are multiple writers to the file at the same time, this is really hard to determine. The first one to call fsync will see any stored error, and the rest get back 0. The processes with open fds may not be associated with one another in any way. They could even be in different containers, so ensuring coordination between all fsync callers is not really an option. One way to remedy this would be to track what file descriptor was used to dirty the file, but that's rather cumbersome and would likely be slow. However, there is a simpler way to improve the semantics here without incurring too much overhead. This set adds an errseq_t to struct address_space, and a corresponding one is added to struct file. Writeback errors are recorded in the mapping's errseq_t, and the one in struct file is used as the "since" value. This changes the semantics of the Linux fsync implementation such that applications can now use it to determine whether there were any writeback errors since fsync(fd) was last called (or since the file was opened in the case of fsync having never been called). Note that those writeback errors may have occurred when writing data that was dirtied via an entirely different fd, but that's the case now with the current mapping_set_error/filemap_check_error infrastructure. This will at least prevent you from getting a false report of success. The new behavior is still consistent with the POSIX spec, and is more reliable for application developers. This patch just adds some basic infrastructure for doing this. Later patches will change the existing code to use this new infrastructure. Signed-off-by: Jeff Layton <jlayton@redhat.com> --- Documentation/filesystems/vfs.txt | 10 +++++++++- fs/open.c | 3 +++ include/linux/fs.h | 24 ++++++++++++++++++++++++ mm/filemap.c | 38 ++++++++++++++++++++++++++++++++++++++ 4 files changed, 74 insertions(+), 1 deletion(-)