Re: blktap: Sync with XCP, dropping zero-copy.

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 11/17/2010 01:57 PM, Daniel Stodden wrote:
> On Wed, 2010-11-17 at 16:02 -0500, Jeremy Fitzhardinge wrote:
>> On 11/17/2010 12:21 PM, Daniel Stodden wrote:
>>> And, like all granted frames, not owning them implies they are not
>>> resolvable via mfn_to_pfn, thereby failing in follow_page, thereby gup()
>>> without the VM_FOREIGN hack.
>> Hm, I see.  Well, I wonder if using _PAGE_SPECIAL would help (it is put
>> on usermode ptes which don't have a backing struct page).  After all,
>> there's no fundamental reason why it would need a pfn; the mfn in the
>> pte is what's actually needed to ultimately generate a DMA descriptor.
> The kernel needs the page structs at least for locking and refcounting.

Yeah.

> There's also a some trickier stuff in there. Like redirtying disk-backed
> user memory after read completion, in case it's been laundered. (So that
> an AIO on unpinned user memory doesn't subsequently get flashed back
> when cycling through swap, if I understood that thing correctly.)
>
> Doesn't apply for blktap (it's all reserved pages). All I mean is: I
> wouldn't exactly see some innocent little dio hack or so shape up in
> there.
>
> Kernel allowing to DMA into a bare pfnmap -- From the platform POV, I'd
> agree. E.g. there's a concept of devices DMA-ing into arbitrary I/O
> memory space, not host memory, on some bus architectures. PCI would come
> to my mind (the old shared medium stuff, unsure about those newfangled
> P-t-P topologies). But not in Linux, so I presently don't see anybody
> upstream bothering to make block-I/O request addressing more forgiving
> than it is.
>
> PAGE_SPECIAL -- to the kernel, that means the opposite: page structs
> which aren't backed by 'real' memory, so gup(), for example, is told to
> fail (how nasty).

It's pfns with no corresponding struct page - it's the pte level
equivalent of VM_PFNMAP at the VMA level.  But you're right that we
can't do without struct pages.

So we're back to needing a way of mapping from a random mfn to a pfn so
we can find the corresponding struct page.  I would be tempted to put a
layer over m2p to allow local m2p mappings to override the global m2p table.

>  In contrast, VM_FOREIGN is non-memory backed by page
> structs.
Yep.

    J