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.
