From: Nadav Amit <namit@xxxxxxxxxx> This patch-set is based on a very small subset of an old RFC (see link below), and intended to avoid TLB flushes when they are not necessary architecturally. Specifically, memory-unprotect using userfaultfd (i.e., using userfaultfd IOCTL) triggers a TLB flush when in fact no architectural data, other than a software flag, is updated. This overhead shows up in my development workload profiles. Instead of tailoring a solution for this specific scenario, it is arguably better to use this opportunity to consolidate the interfaces that are used for TLB batching, avoid the open-coded [inc|dec]_tlb_flush_pending() and use the tlb_[gather|finish]_mmu() interface instead. Avoiding the TLB flushes is done very conservatively (unlike the RFC): 1. According to x86 specifications no flushes are necessary on permission promotion and changes to software bits. 2. Linux does not flush PTEs after the access bit is cleared. I considered the feedback of Andy Lutomirski and Andrew Cooper for the RFC regarding avoiding TLB invalidations when RW is cleared for clean PTEs. Although the bugs they pointed out can be easily addressed, I am concerned since I could not find specifications that explicitly clarify this optimization is valid. -- RFC -> v1: * Do not skip TLB flushes when clearing RW on clean PTEs * Do not defer huge PMD flush as it is already done inline Link: https://lore.kernel.org/lkml/20210131001132.3368247-1-namit@xxxxxxxxxx/ Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Will Deacon <will@xxxxxxxxxx> Cc: Yu Zhao <yuzhao@xxxxxxxxxx> Cc: Nick Piggin <npiggin@xxxxxxxxx> Cc: x86@xxxxxxxxxx Nadav Amit (2): mm/mprotect: use mmu_gather mm/mprotect: do not flush on permission promotion arch/x86/include/asm/tlbflush.h | 40 ++++++++++++++++++++++++++ include/asm-generic/tlb.h | 4 +++ mm/mprotect.c | 51 +++++++++++++++++++-------------- 3 files changed, 73 insertions(+), 22 deletions(-) -- 2.25.1
