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

[Ingo Molnar <mingo@kernel.org>]

* 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.

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.

And if the usage is pure fd based, we don't really have fd-to-fd APIs 
beyond the rarely used splice variants (and even those don't do pure 
cross-IO, they use a pipe as an intermediary), so there's no problem 
to solve I suspect.

Thanks,

	Ingo