Messages in this thread | | | Date | Thu, 22 Feb 2018 04:22:54 -0800 | From | Matthew Wilcox <> | Subject | Re: Use higher-order pages in vmalloc |
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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.
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