Re: [RFC] Better page cache error handling

[Matthew Wilcox]

On Wed, Feb 24, 2021 at 01:38:48PM +0100, Jan Kara wrote:
> > We allocate a page and try to read it.  29 threads pile up waiting
> > for the page lock in filemap_update_page().  The error returned by the
> > original I/O is shared between all 29 waiters as well as being returned
> > to the requesting thread.  The next request for index.html will send
> > another I/O, and more waiters will pile up trying to get the page lock,
> > but at no time will more than 30 threads be waiting for the I/O to fail.
> 
> Interesting idea. It certainly improves current behavior. I just wonder
> whether this isn't a partial solution to a problem and a full solution of
> it would have to go in a different direction? I mean it just seems
> wrong that each reader (let's assume they just won't overlap) has to retry
> the failed IO and wait for the HW to figure out it's not going to work.
> Shouldn't we cache the error state with the page? And I understand that we
> then also have to deal with the problem how to invalidate the error state
> when the block might eventually become readable (for stuff like temporary
> IO failures). That would need some signalling from the driver to the page
> cache, maybe in a form of some error recovery sequence counter or something
> like that. For stuff like iSCSI, multipath, or NBD it could be doable I
> believe...

That felt like a larger change than I wanted to make.  I already have
a few big projects on my plate!

Also, it's not clear to me that the host can necessarily figure out when
a device has fixed an error -- certainly for the three cases you list
it can be done.  I think we'd want a timer to indicate that it's worth
retrying instead of returning the error.

Anyway, that seems like a lot of data to cram into a struct page.  So I
think my proposal is still worth pursuing while waiting for someone to
come up with a perfect solution.