[PATCH v4 0/2] vfs: have syncfs() return error when there are writeback errors

[PATCH v4 0/2] vfs: have syncfs() return error when there are writeback errors

[Jeff Layton]

v4:
- switch to dedicated errseq_t cursor in struct file for syncfs
- drop ioctl for fetching the errseq_t without syncing

This is the fourth posting of this patchset. After thinking about it
more, I think multiplexing file->f_wb_err based on O_PATH open is just
too weird. I think it'd be better if syncfs() "just worked" as expected
no matter what sort of fd you use, or how you multiplex it with fsync.

Also (at least on x86_64) there is currently a 4 byte pad at the end of
the struct so this doesn't end up growing the memory utilization anyway.
Does anyone object to doing this?

I've also dropped the ioctl patch. I have a draft patch to expose that
via fsinfo, but that functionality is really separate from returning an
error to syncfs. We can look at that after the syncfs piece is settled.

Jeff Layton (2):
  vfs: track per-sb writeback errors and report them to syncfs
  buffer: record blockdev write errors in super_block that it backs

 drivers/dax/device.c    |  1 +
 fs/buffer.c             |  2 ++
 fs/file_table.c         |  1 +
 fs/open.c               |  3 +--
 fs/sync.c               |  6 ++++--
 include/linux/fs.h      | 16 ++++++++++++++++
 include/linux/pagemap.h |  5 ++++-
 7 files changed, 29 insertions(+), 5 deletions(-)

-- 
2.24.1

[PATCH v4 1/2] vfs: track per-sb writeback errors and report them to syncfs

[Jeff Layton]

From: Jeff Layton <jlayton@redhat.com>

Usually we suggest that applications call fsync when they want to
ensure that all data written to the file has made it to the backing
store, but that can be inefficient when there are a lot of open
files.

Calling syncfs on the filesystem can be more efficient in some
situations, but the error reporting doesn't currently work the way most
people expect. If a single inode on a filesystem reports a writeback
error, syncfs won't necessarily return an error. syncfs only returns an
error if __sync_blockdev fails, and on some filesystems that's a no-op.

It would be better if syncfs reported an error if there were any writeback
failures. Then applications could call syncfs to see if there are any
errors on any open files, and could then call fsync on all of the other
descriptors to figure out which one failed.

This patch adds a new errseq_t to struct super_block, and has
mapping_set_error also record writeback errors there.

To report those errors, we also need to keep an errseq_t in struct
file to act as a cursor. This patch adds a dedicated field for that
purpose, which slots nicely into 4 bytes of padding at the end of
struct file on x86_64.

An earlier version of this patch used an O_PATH file descriptor to cue
the kernel that the open file should track the superblock error and not
the inode's writeback error.

I think that API is just too weird though. This is simpler and should
make syncfs error reporting "just work" even if someone is multiplexing
fsync and syncfs on the same fds.

Cc: Andres Freund <andres@anarazel.de>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 drivers/dax/device.c    |  1 +
 fs/file_table.c         |  1 +
 fs/open.c               |  3 +--
 fs/sync.c               |  6 ++++--
 include/linux/fs.h      | 16 ++++++++++++++++
 include/linux/pagemap.h |  5 ++++-
 6 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/drivers/dax/device.c b/drivers/dax/device.c
index 1af823b2fe6b..4c0af2eb7e19 100644
--- a/drivers/dax/device.c
+++ b/drivers/dax/device.c
@@ -377,6 +377,7 @@ static int dax_open(struct inode *inode, struct file *filp)
 	inode->i_mapping->a_ops = &dev_dax_aops;
 	filp->f_mapping = inode->i_mapping;
 	filp->f_wb_err = filemap_sample_wb_err(filp->f_mapping);
+	filp->f_sb_err = file_sample_sb_err(filp);
 	filp->private_data = dev_dax;
 	inode->i_flags = S_DAX;
 
diff --git a/fs/file_table.c b/fs/file_table.c
index 30d55c9a1744..676e620948d2 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -198,6 +198,7 @@ static struct file *alloc_file(const struct path *path, int flags,
 	file->f_inode = path->dentry->d_inode;
 	file->f_mapping = path->dentry->d_inode->i_mapping;
 	file->f_wb_err = filemap_sample_wb_err(file->f_mapping);
+	file->f_sb_err = file_sample_sb_err(file);
 	if ((file->f_mode & FMODE_READ) &&
 	     likely(fop->read || fop->read_iter))
 		file->f_mode |= FMODE_CAN_READ;
diff --git a/fs/open.c b/fs/open.c
index 0788b3715731..5007ca287287 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -743,9 +743,8 @@ static int do_dentry_open(struct file *f,
 	path_get(&f->f_path);
 	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_err(f->f_mapping);
+	f->f_sb_err = file_sample_sb_err(f);
 
 	if (unlikely(f->f_flags & O_PATH)) {
 		f->f_mode = FMODE_PATH | FMODE_OPENED;
diff --git a/fs/sync.c b/fs/sync.c
index 4d1ff010bc5a..c6f6f5be5682 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -161,7 +161,7 @@ SYSCALL_DEFINE1(syncfs, int, fd)
 {
 	struct fd f = fdget(fd);
 	struct super_block *sb;
-	int ret;
+	int ret, ret2;
 
 	if (!f.file)
 		return -EBADF;
@@ -171,8 +171,10 @@ SYSCALL_DEFINE1(syncfs, int, fd)
 	ret = sync_filesystem(sb);
 	up_read(&sb->s_umount);
 
+	ret2 = errseq_check_and_advance(&sb->s_wb_err, &f.file->f_sb_err);
+
 	fdput(f);
-	return ret;
+	return ret ? ret : ret2;
 }
 
 /**
diff --git a/include/linux/fs.h b/include/linux/fs.h
index f74a4ee36eb3..d864beeb9360 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -972,6 +972,7 @@ struct file {
 #endif /* #ifdef CONFIG_EPOLL */
 	struct address_space	*f_mapping;
 	errseq_t		f_wb_err;
+	errseq_t		f_sb_err; /* for syncfs */
 } __randomize_layout
   __attribute__((aligned(4)));	/* lest something weird decides that 2 is OK */
 
@@ -1516,6 +1517,9 @@ struct super_block {
 	/* Being remounted read-only */
 	int s_readonly_remount;
 
+	/* per-sb errseq_t for reporting writeback errors via syncfs */
+	errseq_t s_wb_err;
+
 	/* AIO completions deferred from interrupt context */
 	struct workqueue_struct *s_dio_done_wq;
 	struct hlist_head s_pins;
@@ -2827,6 +2831,18 @@ static inline errseq_t filemap_sample_wb_err(struct address_space *mapping)
 	return errseq_sample(&mapping->wb_err);
 }
 
+/**
+ * file_sample_sb_err - sample the current errseq_t to test for later errors
+ * @mapping: mapping to be sampled
+ *
+ * Grab the most current superblock-level errseq_t value for the given
+ * struct file.
+ */
+static inline errseq_t file_sample_sb_err(struct file *file)
+{
+	return errseq_sample(&file->f_path.dentry->d_sb->s_wb_err);
+}
+
 static inline int filemap_nr_thps(struct address_space *mapping)
 {
 #ifdef CONFIG_READ_ONLY_THP_FOR_FS
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index ccb14b6a16b5..897439475315 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -51,7 +51,10 @@ static inline void mapping_set_error(struct address_space *mapping, int error)
 		return;
 
 	/* Record in wb_err for checkers using errseq_t based tracking */
-	filemap_set_wb_err(mapping, error);
+	__filemap_set_wb_err(mapping, error);
+
+	/* Record it in superblock */
+	errseq_set(&mapping->host->i_sb->s_wb_err, error);
 
 	/* Record it in flags for now, for legacy callers */
 	if (error == -ENOSPC)
-- 
2.24.1

[PATCH v4 2/2] buffer: record blockdev write errors in super_block that it backs

[Jeff Layton]

From: Jeff Layton <jlayton@redhat.com>

When syncing out a block device (a'la __sync_blockdev), any error
encountered will only be recorded in the bd_inode's mapping. When the
blockdev contains a filesystem however, we'd like to also record the
error in the super_block that's stored there.

Make mark_buffer_write_io_error also record the error in the
corresponding super_block when a writeback error occurs and the block
device contains a mounted superblock.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/buffer.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/buffer.c b/fs/buffer.c
index b8d28370cfd7..451f1be6e1a4 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1166,6 +1166,8 @@ void mark_buffer_write_io_error(struct buffer_head *bh)
 		mapping_set_error(bh->b_page->mapping, -EIO);
 	if (bh->b_assoc_map)
 		mapping_set_error(bh->b_assoc_map, -EIO);
+	if (bh->b_bdev->bd_super)
+		errseq_set(&bh->b_bdev->bd_super->s_wb_err, -EIO);
 }
 EXPORT_SYMBOL(mark_buffer_write_io_error);
 
-- 
2.24.1

Re: [PATCH v4 0/2] vfs: have syncfs() return error when there are writeback errors

[Jeff Layton]

On Thu, 2020-02-13 at 16:02 -0500, Jeff Layton wrote:
> v4:
> - switch to dedicated errseq_t cursor in struct file for syncfs
> - drop ioctl for fetching the errseq_t without syncing
> 
> This is the fourth posting of this patchset. After thinking about it
> more, I think multiplexing file->f_wb_err based on O_PATH open is just
> too weird. I think it'd be better if syncfs() "just worked" as expected
> no matter what sort of fd you use, or how you multiplex it with fsync.
> 
> Also (at least on x86_64) there is currently a 4 byte pad at the end of
> the struct so this doesn't end up growing the memory utilization anyway.
> Does anyone object to doing this?
> 
> I've also dropped the ioctl patch. I have a draft patch to expose that
> via fsinfo, but that functionality is really separate from returning an
> error to syncfs. We can look at that after the syncfs piece is settled.
> 
> Jeff Layton (2):
>   vfs: track per-sb writeback errors and report them to syncfs
>   buffer: record blockdev write errors in super_block that it backs
> 
>  drivers/dax/device.c    |  1 +
>  fs/buffer.c             |  2 ++
>  fs/file_table.c         |  1 +
>  fs/open.c               |  3 +--
>  fs/sync.c               |  6 ++++--
>  include/linux/fs.h      | 16 ++++++++++++++++
>  include/linux/pagemap.h |  5 ++++-
>  7 files changed, 29 insertions(+), 5 deletions(-)
> 

Hi Al,

Wondering if you've had a chance to look at these yet? I think it makes
sense -- the only part I'm not sure about is adding a field to struct
file. That ends up inside the 4-byte pad at the end on x86_64, so my
hope is that that's not a problem.

If you're too busy at the moment, then maybe Andrew can help shepherd
this in instead?

Thanks,
-- 
Jeff Layton <jlayton@kernel.org>