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. 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? -- 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>
