Re: [dm-devel] [PATCH 0/6] dax poison recovery with RWF_RECOVERY_DATA flag

[Christoph Hellwig <hch@infradead.org>]

On Wed, Oct 27, 2021 at 05:24:51PM -0700, Darrick J. Wong wrote:
> ...so would you happen to know if anyone's working on solving this
> problem for us by putting the memory controller in charge of dealing
> with media errors?

The only one who could know is Intel..

> The trouble is, we really /do/ want to be able to (re)write the failed
> area, and we probably want to try to read whatever we can.  Those are
> reads and writes, not {pre,f}allocation activities.  This is where Dave
> and I arrived at a month ago.
> 
> Unless you'd be ok with a second IO path for recovery where we're
> allowed to be slow?  That would probably have the same user interface
> flag, just a different path into the pmem driver.

Which is fine with me.  If you look at the API here we do have the
RWF_ API, which them maps to the IOMAP API, which maps to the DAX_
API which then gets special casing over three methods.

And while Pavel pointed out that he and Jens are now optimizing for
single branches like this.  I think this actually is silly and it is
not my point.

The point is that the DAX in-kernel API is a mess, and before we make
it even worse we need to sort it first.  What is directly relevant
here is that the copy_from_iter and copy_to_iter APIs do not make
sense.  Most of the DAX API is based around getting a memory mapping
using ->direct_access, it is just the read/write path which is a slow
path that actually uses this.  I have a very WIP patch series to try
to sort this out here:

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dax-devirtualize

But back to this series.  The basic DAX model is that the callers gets a
memory mapping an just works on that, maybe calling a sync after a write
in a few cases.  So any kind of recovery really needs to be able to
work with that model as going forward the copy_to/from_iter path will
be used less and less.  i.e. file systems can and should use
direct_access directly instead of using the block layer implementation
in the pmem driver.  As an example the dm-writecache driver, the pending
bcache nvdimm support and the (horribly and out of tree) nova file systems
won't even use this path.  We need to find a way to support recovery
for them.  And overloading it over the read/write path which is not
the main path for DAX, but the absolutely fast path for 99% of the
kernel users is a horrible idea.

So how can we work around the horrible nvdimm design for data recovery
in a way that:

   a) actually works with the intended direct memory map use case
   b) doesn't really affect the normal kernel too much

?