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

[Dan Williams]

On Thu, Nov 4, 2021 at 11:34 AM Jane Chu <jane.chu@xxxxxxxxxx> wrote:
>
> Thanks for the enlightening discussion here, it's so helpful!
>
> Please allow me to recap what I've caught up so far -
>
> 1. recovery write at page boundary due to NP setting in poisoned
>     page to prevent undesirable prefetching
> 2. single interface to perform 3 tasks:
>       { clear-poison, update error-list, write }
>     such as an API in pmem driver.
>     For CPUs that support MOVEDIR64B, the 'clear-poison' and 'write'
>     task can be combined (would need something different from the
>     existing _copy_mcsafe though) and 'update error-list' follows
>     closely behind;
>     For CPUs that rely on firmware call to clear posion, the existing
>     pmem_clear_poison() can be used, followed by the 'write' task.
> 3. if user isn't given RWF_RECOVERY_FLAG flag, then dax recovery
>     would be automatic for a write if range is page aligned;
>     otherwise, the write fails with EIO as usual.
>     Also, user mustn't have punched out the poisoned page in which
>     case poison repairing will be a lot more complicated.
> 4. desirable to fetch as much data as possible from a poisoned range.
>
> If this understanding is in the right direction, then I'd like to
> propose below changes to
>    dax_direct_access(), dax_copy_to/from_iter(), pmem_copy_to/from_iter()
>    and the dm layer copy_to/from_iter, dax_iomap_iter().
>
> 1. dax_iomap_iter() rely on dax_direct_access() to decide whether there
>     is likely media error: if the API without DAX_F_RECOVERY returns
>     -EIO, then switch to recovery-read/write code.  In recovery code,
>     supply DAX_F_RECOVERY to dax_direct_access() in order to obtain
>     'kaddr', and then call dax_copy_to/from_iter() with DAX_F_RECOVERY.

I like it. It allows for an atomic write+clear implementation on
capable platforms and coordinates with potentially unmapped pages. The
best of both worlds from the dax_clear_poison() proposal and my "take
a fault and do a slow-path copy".

> 2. the _copy_to/from_iter implementation would be largely the same
>     as in my recent patch, but some changes in Christoph's
>     'dax-devirtualize' maybe kept, such as DAX_F_VIRTUAL, obviously
>     virtual devices don't have the ability to clear poison, so no need
>     to complicate them.  And this also means that not every endpoint
>     dax device has to provide dax_op.copy_to/from_iter, they may use the
>     default.

Did I miss this series or are you talking about this one?
https://lore.kernel.org/all/20211018044054.1779424-1-hch@xxxxxx/

> I'm not sure about nova and others, if they use different 'write' other
> than via iomap, does that mean there will be need for a new set of
> dax_op for their read/write?

No, they're out-of-tree they'll adjust to the same interface that xfs
and ext4 are using when/if they go upstream.

> the 3-in-1 binding would always be
> required though. Maybe that'll be an ongoing discussion?

Yeah, let's cross that bridge when we come to it.

> Comments? Suggestions?

It sounds great to me!