Re: Use higher-order pages in vmalloc

[Matthew Wilcox]

On Thu, Feb 22, 2018 at 07:59:43AM +0100, Michal Hocko wrote:
> On Wed 21-02-18 09:01:29, Matthew Wilcox wrote:
> > 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.

I was wondering about that possibility.  If we want to migrate a page
then we have to shoot down the PTE across all CPUs, copy the data to the
new page, and insert the new PTE.  Copying 4kB doesn't take long; if you
have 12GB/s (current example on Wikipedia: dual-channel memory and one
DDR2-800 module per channel gives a theoretical bandwidth of 12.8GB/s)
then we should be able to copy a page in 666ns).  So there's no problem
holding a spinlock for it.

But we can't handle a fault in vmalloc space today.  It's handled in
arch-specific code, see vmalloc_fault() in arch/x86/mm/fault.c
If we're going to do this, it'll have to be something arches opt into
because I'm not taking on the job of fixing every architecture!

> Maybe we should consider kvmalloc for the kernel stack?

We'd lose the guard page, so it'd have to be something we let the
sysadmin decide to do.