Hi Andrea,

On Mon, Aug 22, 2011 at 2:33 PM, Andrea Arcangeli [off-list ref] wrote:

So this solution:

1) should allow the working set estimation code to keep doing its
get_page_unless_zero() without any change (you'll still have to modify
it to check if you got a THP page etc... but you won't risk to get any
tail page anymore). Maybe it still needs some non trivial thought
about the changes but not anymore about tail pages refcounting screwups.

2) no change to all existing get_page_unless_zero() is required, so
this should fix the radix tree speculative page lookup too.

3) no RCU new feature is needed

Adding Paul McKenney so he won't spend too much time on RCU cookie
feature until there is a firmer user...

4) get_page was actually called by direct-io as my debug
instrumentation I wrote to test these changes noticed it so I fixed
that too

Looks like this scheme will work. I'm off in Yosemite for a few days
with my family, but I should be able to review this more thoroughly on
Thursday.

From a few-minutes look, I have a few minor concerns:

- When splitting THP pages, the old tail refcount will be visible as
the _mapcount for a short while after PageTail is cleared; not clear
yet to me if there are unintended side effects to that;
- (not a concern, but an opportunity) when splitting pages, there are
two atomic adds to the tail _count field, while we know the initial
value is 0. Why not just one straight assignment ? Similarly, the
adjustments to page head count could be added into a local variable
and the page head count could be updated once after all tail pages
have been split off.
- Not sure if we could/should add assertions to make sure people call
the right get_page variant.

The other question I have is about the use of the pagemap.h RCU
protocol for eventual page count stability. With your proposal, this
would now affect only head pages, so THP splitting is fine :) . I'm
not sure who else might use that protocol, but it looks like we should
either make all get_pages_unless_zero call sites follow it (if the
protocol matters to someone) or none (if the protocol turns out to be
obsolete).

Sorry for the incomplete reply, I'll have a better one by Thursday :)

Thanks,

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.

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