Re: [RFC PATCH 0/4] fs: introduce new writeback error tracking infrastructure and convert ext4 to use it

[Matthew Wilcox <willy@infradead.org>]

On Wed, Apr 05, 2017 at 03:49:52PM -0400, Jeff Layton wrote:
> > That only gives us 20 bits of counter, but I think that's enough.
> 
> 2^20 is 1048576, which seems a little small to me.
> 
> We may end up bumping the counter on every failed I/O. How fast can we
> generate 1M failed I/Os? :)

So there's a one-in-a-million chance of missing a failed I/O ... if
we're generating lots of errors, the next time the app calls fsync(),
it'll notice the other million times we've hit the problem :-)

> Actually...we could put this field in the inode instead of the mapping.
> I know we've traditionally tracked this in the mapping, but is that
> required here?
> 
> If we put this field in the inode then perhaps we can union it with
> something and mitigate the cost of a larger counter...maybe in the
> i_pipe union? I don't think S_ISREG inodes use anything in there, do
> they?

But writeback isn't just done on ISREG inodes, but also on S_ISBLK inodes,
which use i_bdev (right?)

Another possibility is to move this out of the address_space and into
either the super_block or the backing_device_info.  Errors don't tend
to be constrained to a single file but affect the entire filesystem,
or even multiple filesystems if you have a partitioned block device ...