On Thu, May 7, 2015 at 10:43 AM, Linus Torvalds
On Thu, May 7, 2015 at 9:03 AM, Dan Williams
> 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.
Ok. Although __pfn_t also stores pfn values directly which will
consume those 2 bits so we'll need to shift pfns up when storing.