Re: Use higher-order pages in vmalloc

[Michal Hocko]

On Thu 22-02-18 19:01:35, Andy Lutomirski wrote:
> On Thu, Feb 22, 2018 at 1:36 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > On Thu 22-02-18 04:22:54, Matthew Wilcox wrote:
> >> 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!
> >
> > yes.
> 
> On x86, if you shoot down the PTE for the current stack, you're dead.
> vmalloc_fault() might not even be called.  Instead we hit
> do_double_fault(), and the manual warns extremely strongly against
> trying to recover, and, in this case, I agree with the SDM.  If you
> actually want this to work, there needs to be a special IPI broadcast
> to the task in question (with appropriate synchronization) that calls
> magic arch code that does the switcheroo.

Why cannot we use the pte swap entry trick also for vmalloc migration.
I haven't explored this path at all, to be honest.

> Didn't someone (Christoph?) have a patch to teach the page allocator
> to give high-order allocations if available and otherwise fall back to
> low order?

Do you mean kvmalloc?

-- 
Michal Hocko
SUSE Labs