Re: [PATCH v3 1/5] mm: introduce MADV_COLD

[Michal Hocko <mhocko@kernel.org>]

On Thu 27-06-19 07:36:50, Dave Hansen wrote:
[...]
> For MADV_COLD, if we defined it like this, I think we could use it for
> both purposes (demotion and LRU movement):
> 
> 	Pages in the specified regions will be treated as less-recently-
> 	accessed compared to pages in the system with similar access
> 	frequencies.  In contrast to MADV_DONTNEED, the contents of the

you meant s@MADV_DONTNEED@MADV_FREE@ I suppose

> 	region are preserved.
> 
> It would be nice not to talk about reclaim at all since we're not
> promising reclaim per se.

Well, I guess this is just an implementation detail. MADV_FREE is really
only about aging. It is up to the kernel what to do during the reclaim
and the advice doesn't and shouldn't make any difference here.

Now MADV_PAGEOUT would be more tricky in that direction because it
defines an immediate action to page out the range. I do understand your
argument about NUMA unaware applications which might want to get
something like MADV_DEMOTE which would move a page to a secondary memory
(whatever that is) but I think this is asking for its own madvise.
MADV_PAGEOUT has a quite simple semnatic - move to the backing storage -
and I would rather not make it more complex.
-- 
Michal Hocko
SUSE Labs