On Oct 25, 2022, at 6:06 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > if (!force_flush && !tlb->fullmm && details && > + details->zap_flags & ZAP_FLAG_FORCE_FLUSH) > + force_flush = 1; Isn’t it too big of a hammer? At the same time, the whole reasoning about TLB flushes is not getting any simpler. We had cases in which MADV_DONTNEED and another concurrent operation that effectively zapped PTEs (e.g., another MADV_DONTNEED) caused the zap_pte_range() to skip entries since pte_none() was true. To resolve these cases we relied on tlb_finish_mmu() to flush the range when needed (i.e., flush the whole range when mm_tlb_flush_nested()). Now, I do not have a specific broken scenario in mind following this change, but it is all sounds to me a bit dangerous and at same time can potentially introduce new overheads. One alternative may be using mm_tlb_flush_pending() when setting a new PTE to check for pending flushes and flushing the TLB if that is the case. This is somewhat similar to what ptep_clear_flush() does. Anyhow, I guess this might induce some overheads. As noted before, it is possible to track pending TLB flushes in VMA/page-table granularity, with different tradeoffs of overheads.
