Re: [PATCH 03/62] mm: Split slab into its own type

[Matthew Wilcox <willy@infradead.org>]

On Wed, Oct 13, 2021 at 02:08:57PM -0400, Johannes Weiner wrote:
> Btw, I think slab_nid() is an interesting thing when it comes to page
> polymorphy. We want to know the nid for all sorts of memory types:
> slab, file, anon, buddy etc. In the goal of distilling page down to
> the fewest number of bytes, this is probably something that should
> remain in the page rather than be replicated in all subtypes.

Oh, this is a really interesting point.

Node ID is typically 10 bits (I checked Debian & Oracle configs for
various architectures).  That's far more than we can store in the bottom
bits of a single word, and it's even a good chunk of a second word.

I was assuming that, for the page allocator's memory descriptor and for
that of many allocators (such as slab), it would be stored *somewhere*
in the memory descriptor.  It wouldn't necessarily have to be the same
place for all memory descriptors, and maybe (if it's accessed rarely),
we delegate finding it to the page allocator's knowledge.

But not all memory descriptors want/need/can know this.  For example,
vmalloc() might well spread its memory across multiple nodes.  As long
as we can restore the node assignment again once the pages are vfree(),
there's no particular need for the vmalloc memory descriptor to know
what node an individual page came from (and the concept of asking
vmalloc what node a particular allocation came from is potentially
nonsense, unless somebody used vmalloc_node() or one of the variants).

Not sure there's an obviously right answer here.  I was assuming that at
first we'd enforce memdesc->flags being the first word of every memory
descriptor and so we could keep passing page->flags around.  That could
then change later, but it'd be a good first step?