On Fri, 2018-02-23 at 13:13 +0100, Michal Hocko wrote: > 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? I sent something last year but had not finished the patch series :/ https://marc.info/?l=linux-kernel&m=148233423610544&w=2 -- 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>
