Re: Use higher-order pages in vmalloc

[Michal Hocko]

On Wed 21-02-18 09:01:29, Matthew Wilcox wrote:
> On Wed, Feb 21, 2018 at 08:16:22AM -0800, Dave Hansen wrote:
> > On 02/21/2018 07:42 AM, Matthew Wilcox wrote:
> > > This prompted me to write a patch I've been meaning to do for a while,
> > > allocating large pages if they're available to satisfy vmalloc.  I thought
> > > it would save on touching multiple struct pages, but it turns out that
> > > the checking code we currently have in the free_pages path requires you
> > > to have initialised all of the tail pages (maybe we can make that code
> > > conditional ...)
> > 
> > What the concept here?  If we can use high-order pages for vmalloc() at
> > the moment, we *should* use them?
> 
> Right.  It helps with fragmentation if we can keep higher-order
> allocations together.

Hmm, wouldn't it help if we made vmalloc pages migrateable instead? That
would help the compaction and get us to a lower fragmentation longterm
without playing tricks in the allocation path.

> > One of the coolest things about vmalloc() is that it can do large
> > allocations without consuming large (high-order) pages, so it has very
> > few side-effects compared to doing a bunch of order-0 allocations.  This
> > patch seems to propose removing that cool thing.  Even trying the
> > high-order allocation could kick off a bunch of reclaim and compaction
> > that was not there previously.
> 
> Yes, that's one of the debatable things.  It'd be nice to have a GFP
> flag that stopped after calling get_page_from_freelist() and didn't try
> to do compaction or reclaim.

GFP_NOWAIT, you mean?

> > If you could take this an only _opportunistically_ allocate large pages,
> > it could be a more universal win.  You could try to make sure that no
> > compaction or reclaim is done for the large allocation.  Or, maybe you
> > only try it if there are *only* high-order pages in the allocator that
> > would have been broken down into order-0 *anyway*.
> > 
> > I'm not sure it's worth it, though.  I don't see a lot of folks
> > complaining about vmalloc()'s speed or TLB impact.
> 
> No, I'm not sure it's worth it either, although Konstantin's mail
> suggesting improvements in fork speed were possible by avoiding vmalloc
> reminded me that I'd been meaning to give this a try.

Maybe we should consider kvmalloc for the kernel stack?
-- 
Michal Hocko
SUSE Labs