On Tue, May 16, 2023 at 04:07:17PM +0800, Baoquan He wrote: > On 05/16/23 at 08:40am, Thomas Gleixner wrote: > > On Tue, May 16 2023 at 10:26, Baoquan He wrote: > > > On 05/15/23 at 08:17pm, Uladzislau Rezki wrote: > > >> For systems which lack a full TLB flush and to flush a long range is > > >> a problem(it takes time), probably we can flush VA one by one. Because > > >> currently we calculate a flush range [min:max] and that range includes > > >> the space that might not be mapped at all. Like below: > > > > > > It's fine if we only calculate a flush range of [min:max] with VA. In > > > vm_reset_perms(), it calculates the flush range with the impacted direct > > > mapping range, then merge it with VA's range. That looks really strange > > > and surprising. If the vm->pages[] are got from a lower part of physical > > > memory, the final merged flush will span tremendous range. Wondering why > > > we need merge the direct map range with VA range, then do flush. Not > > > sure if I misunderstand it. > > > > So what happens on this BPF teardown is: > > > > The vfree(8k) ends up flushing 3 entries. The actual vmalloc part (2) and > > one extra which is in the direct map. I haven't verified that yet, but I > > assume it's the alias of one of the vmalloc'ed pages. > > It looks like the reason. As Uladzislau pointed out, ARCH-es may > have full TLB flush, so won't get trouble from the merged flush > in the calculated [min:max] way, e.g arm64 and x86's flush_tlb_kernel_range(). > However, arm32 seems lacking the ability of full TLB flash. If agreed, I > can make a draft patch to do the flush for direct map and VA seperately, > see if it works. The question IMHO is not so much whether there's a full-TLB flush available, but whether it is appropriate to use it. If we're only wanting to flush a small number of TLB entries but over a sparse range (which seems to be Thomas' situation), does it make any sense to flush all TLB entries? I don't think it does, but it depends how often this occurs. If we're doing it on a regular basis because of some workload, then that workload suffers. If it's a rare event then maybe that's okay to do. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!
