Re: [PATCH v2 00/10] evacuate struct page from the block layer, introduce __pfn_t

[Dan Williams <dan.j.williams@intel.com>]

On Thu, May 7, 2015 at 7:42 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Ingo Molnar <mingo@kernel.org> wrote:
>
>> [...]
>>
>> For anything more complex, that maps any of this storage to
>> user-space, or exposes it to higher level struct page based APIs,
>> etc., where references matter and it's more of a cache with
>> potentially multiple users, not an IO space, the natural API is
>> struct page.
>
> Let me walk back on this:
>
>> I'd say that this particular series mostly addresses the 'pfn as
>> sector_t' side of the equation, where persistent memory is IO space,
>> not memory space, and as such it is the more natural and thus also
>> the cheaper/faster approach.
>
> ... but that does not appear to be the case: this series replaces a
> 'struct page' interface with a pure pfn interface for the express
> purpose of being able to DMA to/from 'memory areas' that are not
> struct page backed.
>
>> Linus probably disagrees? :-)
>
> [ and he'd disagree rightfully ;-) ]
>
> So what this patch set tries to achieve is (sector_t -> sector_t) IO
> between storage devices (i.e. a rare and somewhat weird usecase), and
> does it by squeezing one device's storage address into our formerly
> struct page backed descriptor, via a pfn.
>
> That looks like a layering violation and a mistake to me. If we want
> to do direct (sector_t -> sector_t) IO, with no serialization worries,
> it should have its own (simple) API - which things like hierarchical
> RAID or RDMA APIs could use.

I'm wrapped around the idea that __pfn_t *is* that simple api for the
tiered storage driver use case.  For RDMA I think we need struct page
because I assume that would be coordinated through a filesystem an
truncate() is back in play.

What does an alternative API look like?

> If what we want to do is to support say an mmap() of a file on
> persistent storage, and then read() into that file from another device
> via DMA, then I think we should have allocated struct page backing at
> mmap() time already, and all regular syscall APIs would 'just work'
> from that point on - far above what page-less, pfn-based APIs can do.
>
> The temporary struct page backing can then be freed at munmap() time.

Yes, passing around mmap()'d (DAX) persistent memory will need more
than a __pfn_t.