Re: [RFC v3 0/5] Transparent on-demand struct page initialization embedded in the buddy allocator

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, Aug 12, 2013 at 2:54 PM, Nathan Zimmer <nzimmer@sgi.com> wrote:
>
> As far as extra overhead. We incur an extra function call to
> ensure_page_is_initialized but that is only really expensive when we find
> uninitialized pages, otherwise it is a flag check once every PTRS_PER_PMD.
> To get a better feel for this we ran two quick tests.

Sorry for coming into this late and for this last version of the
patch, but I have to say that I'd *much* rather see this delayed
initialization using another data structure than hooking into the
basic page allocation ones..

I understand that you want to do delayed initialization on some TB+
memory machines, but what I don't understand is why it has to be done
when the pages have already been added to the memory management free
list.

Could we not do this much simpler: make the early boot insert the
first few gigs of memory (initialized) synchronously into the free
lists, and then have a background thread that goes through the rest?

That way the MM layer would never see the uninitialized pages.

And I bet that *nobody* cares if you "only" have a few gigs of ram
during the first few minutes of boot, and you mysteriously end up
getting more and more memory for a while until all the RAM has been
initialized.

IOW, just don't call __free_pages_bootmem() on all the pages al at
once. If we have to remove a few __init markers to be able to do some
of it later, does anybody really care?

I really really dislike this "let's check if memory is initialized at
runtime" approach.

           Linus

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