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

[Linus Torvalds]

On Thu, May 7, 2015 at 9:03 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> Ok, I'll keep thinking about this and come back when we have a better
> story about passing mmap'd persistent memory around in userspace.

Ok. And if we do decide to go with your kind of "__pfn" type, I'd
probably prefer that we encode the type in the low bits of the word
rather than compare against PAGE_OFFSET. On some architectures
PAGE_OFFSET is zero (admittedly probably not ones you'd care about),
but even on x86 it's a *lot* cheaper to test the low bit than it is to
compare against a big constant.

We know "struct page *" is supposed to be at least aligned to at least
"unsigned long", so you'd have two bits of type information (and we
could easily make it three). With "0" being a real pointer, so that
you can use the pointer itself without masking.

And the "hide type in low bits of pointer" is something we've done
quite a lot, so it's more "kernel coding style" anyway.

                            Linus
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