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

[Ingo Molnar <mingo@kernel.org>]

* Dave Hansen <dave.hansen@linux.intel.com> wrote:

> On 05/07/2015 10:42 AM, Dan Williams wrote:
> > On Thu, May 7, 2015 at 10:36 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >> * Dan Williams <dan.j.williams@intel.com> wrote:
> >>
> >> So is there anything fundamentally wrong about creating struct 
> >> page backing at mmap() time (and making sure aliased mmaps share 
> >> struct page arrays)?
> > 
> > Something like "get_user_pages() triggers memory hotplug for 
> > persistent memory", so they are actual real struct pages?  Can we 
> > do memory hotplug at that granularity?
> 
> We've traditionally limited them to SECTION_SIZE granularity, which 
> is 128MB IIRC.  There are also assumptions in places that you can do 
> page++ within a MAX_ORDER block if !CONFIG_HOLES_IN_ZONE.

I really don't think that's very practical: memory hotplug is slow, 
it's really not on the same abstraction level as mmap(), and the zone 
data structures are also fundamentally very coarse: not just because 
RAM ranges are huge, but also so that the pfn->page transformation 
stays relatively simple and fast.

> But, in all practicality, a lot of those places are in code like the 
> buddy allocator.  If your PTEs all have _PAGE_SPECIAL set and we're 
> not ever expecting these fake 'struct page's to hit these code 
> paths, it probably doesn't matter.
> 
> You can probably get away with just allocating PAGE_SIZE worth of 
> 'struct page' (which is 64) and mapping it in to vmemmap[].  The 
> worst case is that you'll eat 1 page of space for each outstanding 
> page of I/O.  That's a lot better than 2MB of temporary 'struct 
> page' space per page of I/O that it would take with a traditional 
> hotplug operation.

So I think the main value of struct page is if everyone on the system 
sees the same struct page for the same pfn - not just the temporary IO 
instance.

The idea of having very temporary struct page arrays misses the point 
I think: if struct page is used as essentially an IO sglist then most 
of the synchronization properties are lost: then we might as well use 
the real deal in that case and skip the dynamic allocation and use 
pfns directly and avoid the dynamic allocation overhead.

Stable, global page-struct descriptors are a given for real RAM, where 
we allocate a struct page for every page in nice, large, mostly linear 
arrays.

We'd really need that for pmem too, to get the full power of struct 
page: and that means allocating them in nice, large, predictable 
places - such as on the device itself ...

It might even be 'scattered' across the device, with 64 byte struct 
page size we can pack 64 descriptors into a single page, so every 65 
pages we could have a page-struct page.

Finding a pmem page's struct page would thus involve rounding it 
modulo 65 and reading that page.

The problem with that is fourfold:

 - that we now turn a very kernel internal API and data structure into 
   an ABI. If struct page grows beyond 64 bytes it's a problem.

 - on bootup (or device discovery time) we'd have to initialize all 
   the page structs. We could probably do this in a hierarchical way, 
   by dividing continuous pmem ranges into power-of-two groups of 
   blocks, and organizing them like the buddy allocator does.

 - 1.5% of storage space lost.

 - will wear-leveling properly migrate these 'hot' pages around?

The alternative would be some global interval-rbtree of struct page 
backed pmem ranges.

Beyond the synchronization problems of such a data structure (which 
looks like a nightmare) I don't think it's even feasible: especially 
if there's a filesystem on the pmem device then the block allocations 
could be physically fragmented (and there's no fundamental reason why 
they couldn't be fragmented), so a continuous mmap() of a file on it 
will yield wildly fragmented device-pfn ranges, exploding the rbtree. 
Think 1 million node interval-rbtree with an average depth of 20: 
cachemiss country for even simple lookups - not to mention the 
freeing/recycling complexity of unused struct pages to not allow it to 
grow too large.

I might be wrong though about all this :)

Thanks,

	Ingo