"Huang, Ying" <ying.huang@xxxxxxxxx> writes: > Peter Xu <peterx@xxxxxxxxxx> writes: > >> On Thu, Aug 18, 2022 at 02:34:45PM +0800, Huang, Ying wrote: >>> > In this specific case, the only way to do safe tlb batching in my mind is: >>> > >>> > pte_offset_map_lock(); >>> > arch_enter_lazy_mmu_mode(); >>> > // If any pending tlb, do it now >>> > if (mm_tlb_flush_pending()) >>> > flush_tlb_range(vma, start, end); >>> > else >>> > flush_tlb_batched_pending(); >>> >>> I don't think we need the above 4 lines. Because we will flush TLB >>> before we access the pages. I agree. For migration the TLB flush is only important if the PTE is present, and in that case we do a TLB flush anyway. >> Could you elaborate? > > As you have said below, we don't use non-present PTEs and flush present > PTEs before we access the pages. > >>> Can you find any issue if we don't use the above 4 lines? >> >> It seems okay to me to leave stall tlb at least within the scope of this >> function. It only collects present ptes and flush propoerly for them. I >> don't quickly see any other implications to other not touched ptes - unlike >> e.g. mprotect(), there's a strong barrier of not allowing further write >> after mprotect() returns. > > Yes. I think so too. > >> Still I don't know whether there'll be any side effect of having stall tlbs >> in !present ptes because I'm not familiar enough with the private dev swap >> migration code. But I think having them will be safe, even if redundant. What side-effect were you thinking of? I don't see any issue with not TLB flushing stale device-private TLBs prior to the migration because they're not accessible anyway and shouldn't be in any TLB. > I don't think it's a good idea to be redundant. That may hide the real > issue. > > Best Regards, > Huang, Ying Thanks all for the discussion. Having done some more reading I agree that it's safe to assume HW dirty bits are write-through, so will remove the ptep_clear_flush() and use ptep_get_and_clear() instead. Will split out the TLB flushing fix into a separate patch in this series. - Alistair
