• Jeff Layton's avatar
    fs: new infrastructure for writeback error handling and reporting · 5660e13d
    Jeff Layton authored
    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, and ensures that the f_wb_err "cursor"
    is properly set when a file is opened. Later patches will change the
    existing code to use this new infrastructure for reporting errors at
    fsync time.
    Signed-off-by: default avatarJeff Layton <jlayton@redhat.com>
    Reviewed-by: default avatarJan Kara <jack@suse.cz>
    5660e13d
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