Date: Tue, 4 Apr 2017 10:09:09 -0700
From: Matthew Wilcox <willy@infradead.org>
To: Jeff Layton <jlayton@redhat.com>
Cc: NeilBrown <neilb@suse.com>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-ext4@vger.kernel.org,
        akpm@linux-foundation.org, tytso@mit.edu, jack@suse.cz
Subject: Re: [RFC PATCH 0/4] fs: introduce new writeback error tracking
 infrastructure and convert ext4 to use it
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On Tue, Apr 04, 2017 at 12:25:46PM -0400, Jeff Layton wrote:
> That said, I think giving more specific errors where we can is useful.
> When your program is erroring out and writing 'I/O error' to the logs,
> then how much time will your admins burn before they figure out that it
> really failed because the filesystem was full?

df is one of the first things I check ... a few years ago, I also learned
to check df -i ... ;-)

Anyway, given the decision to simply report the last error lets us do this
implementation:

void filemap_set_wb_error(struct address_space *mapping, int err)
{
	struct inode *inode = mapping->host;
	unsigned int wb_err;

	if (!err)
		return;
	/*
	 * This should be called with the error code that we want to return
	 * on fsync. Thus, it should always be <= 0.
	 */
	WARN_ON(err > 0 || err < -MAX_ERRNO);

	spin_lock(&inode->i_lock);
	wb_err = ((mapping->wb_err & ~MAX_ERRNO) + (1 << 12)) | -err;
	WRITE_ONCE(mapping->wb_err, wb_err);
	spin_unlock(&inode->i_lock);
}

int filemap_report_wb_error(struct file *file)
{
	struct inode *inode = file_inode(file);
	unsigned int wb_err = READ_ONCE(mapping->wb_err);

	if (file->f_wb_err == wb_err)
		return 0;
	return -(wb_err & 4095);
}

That only gives us 20 bits of counter, but I think that's enough.