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

[Dan Williams]

On Wed, May 6, 2015 at 3:10 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, May 6, 2015 at 1:04 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>>
>> The motivation for this change is persistent memory and the desire to
>> use it not only via the pmem driver, but also as a memory target for I/O
>> (DAX, O_DIRECT, DMA, RDMA, etc) in other parts of the kernel.
>
> I detest this approach.
>

Hmm, yes, I can't argue against "put the onus on odd behavior where it
belongs."...

> I'd much rather go exactly the other way around, and do the dynamic
> "struct page" instead.
>
> Add a flag to "struct page"

Ok, given I had already precluded 32-bit systems in this __pfn_t
approach we should have flag space for this on 64-bit.

> to mark it as a fake entry and teach
> "page_to_pfn()" to look up the actual pfn some way (that union tha
> contains "index" looks like a good target to also contain 'pfn', for
> example).
>
> Especially if this is mainly for persistent storage, we'll never have
> issues with worrying about writing it back under memory pressure, so
> allocating a "struct page" for these things shouldn't be a problem.
> There's likely only a few paths that actually generate IO for those
> things.
>
> In other words, I'd really like our basic infrastructure to be for the
> *normal* case, and the "struct page" is about so much more than just
> "what's the target for IO". For normal IO, "struct page" is also what
> serializes the IO so that you have a consistent view of the end
> result, and there's obviously the reference count there too. So I
> really *really* think that "struct page" is the better entity for
> describing the actual IO, because it's the common and the generic
> thing, while a "pfn" is not actually *enough* for IO in general, and
> you now end up having to look up the "struct page" for the locking and
> refcounting etc.
>
> If you go the other way, and instead generate a "struct page" from the
> pfn for the few cases that need it, you put the onus on odd behavior
> where it belongs.
>
> Yes, it might not be any simpler in the end, but I think it would be
> conceptually much better.

Conceptually better, but certainly more difficult to audit if the fake
struct page is initialized in a subtle way that breaks when/if it
leaks to some unwitting context.  The one benefit I may need to
concede is a mechanism to opt-in to handle these fake pages to the few
paths that know what they are doing.  That was easy with __pfn_t, but
a struct page can go silently almost anywhere.  Certainly nothing is
prepared a for a given struct page pointer to change the pfn it points
to on the fly, which I think is what we would end up doing for
something like a raid cache.  Keep a pool of struct pages around and
point them at persistent memory pfns while I/O is in flight.
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