Re: [PATCH v35 1/5] mm: support to get hints of free page blocks

[Michal Hocko]

On Wed 11-07-18 18:52:45, Wei Wang wrote:
> On 07/11/2018 05:21 PM, Michal Hocko wrote:
> > On Tue 10-07-18 18:44:34, Linus Torvalds wrote:
> > [...]
> > > That was what I tried to encourage with actually removing the pages
> > > form the page list. That would be an _incremental_ interface. You can
> > > remove MAX_ORDER-1 pages one by one (or a hundred at a time), and mark
> > > them free for ballooning that way. And if you still feel you have tons
> > > of free memory, just continue removing more pages from the free list.
> > We already have an interface for that. alloc_pages(GFP_NOWAIT, MAX_ORDER -1).
> > So why do we need any array based interface?
> 
> Yes, I'm trying to get free pages directly via alloc_pages, so there will be
> no new mm APIs.

OK. The above was just a rough example. In fact you would need a more
complex gfp mask. I assume you only want to balloon only memory directly
usable by the kernel so it will be
	(GFP_KERNEL | __GFP_NOWARN) & ~__GFP_RECLAIM

> I plan to let free page allocation stop when the remaining system free
> memory becomes close to min_free_kbytes (prevent swapping).

~__GFP_RECLAIM will make sure you are allocate as long as there is any
memory without reclaim. It will not even poke the kswapd to do the
background work. So I do not think you would need much more than that.

But let me note that I am not really convinced how this (or previous)
approach will really work in most workloads. We tend to cache heavily so
there is rarely any memory free.

-- 
Michal Hocko
SUSE Labs