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

[Dan Williams]

On Thu, May 7, 2015 at 10:43 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 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.

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.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/