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. > > 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. ohh, right, I forgot about the guard page. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
