On Wed, Jul 29, 2015 at 6:59 AM, Vladimir Davydov [off-list ref]
wrote:

quoted

I guess the primary reason to rely on the pfn rather than the LRU walk,
which would be more targeted (especially for memcg cases), is that we
cannot hold lru lock for the whole LRU walk and we cannot continue
walking after the lock is dropped. Maybe we can try to address that
instead? I do not think this is easy to achieve but have you considered
that as an option?

Yes, I have, and I've come to a conclusion it's not doable, because LRU
lists can be constantly rotating at an arbitrary rate. If you have an
idea in mind how this could be done, please share.

Speaking of LRU-vs-PFN walk, iterating over PFNs has its own advantages:
 - You can distribute a walk in time to avoid CPU bursts.
 - You are free to parallelize the scanner as you wish to decrease the
   scan time.

There is a third way: one could go through every MM in the system and scan
their page tables. Doing things that way turns out to be generally faster
than scanning by physical address, because you don't have to go through
RMAP for every page. But, you end up needing to take the mmap_sem lock of
every MM (in turn) while scanning them, and that degrades quickly under
memory load, which is exactly when you most need this feature. So, scan by
address is still what we use here.

My only concern about the interface is that it exposes the fact that the
scan is done by address - if the interface only showed per-memcg totals, it
would make it possible to change the implementation underneath if we
somehow figure out how to work around the mmap_sem issue in the future. I
don't think that is necessarily a blocker but this is something to keep in
mind IMO.

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