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 -- 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>
