RE: [PATCH 2/5] pstore: Introduce write_buf backend callback

[Luck, Tony]

> Hrm, I thought the point of having pstore.buf pre-mapped was to allow
> Oopses to be able to write directly to it without needing to hit any
> additional kernel code. Maybe I'm misunderstanding this change,
> though. I'd like to see Tony's opinion on it.

Yes - the ERST backend needs to have a bunch of header ugliness (with
special UUIDs) at the front of the buffer that is stored to non-volatile
storage. So it allocates its own buffer with all that junk, and then
passes the address of the plain data portion of the buffer on to the
pstore layer.

As we add more backends, it might be that this is only applicable to
ERST, and so it might make sense to have it copy the data from some
other buffer into its specially crafted one.  But we should not lose
the "no allocations" property ... everything needed should be pre-allocated
so we don't have to try to allocate any memory during a panic.

-Tony
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