Re: + mm-mmu_gather-remove-__tlb_reset_range-for-force-flush.patch added to -mm tree

Peter Zijlstra
2019-05-27T11:02:25Z

On Tue, May 21, 2019 at 04:18:33PM -0700, akpm@linux-foundation.org wrote:
> --- a/mm/mmu_gather.c~mm-mmu_gather-remove-__tlb_reset_range-for-force-flush
> +++ a/mm/mmu_gather.c
> @@ -245,14 +245,28 @@ void tlb_finish_mmu(struct mmu_gather *t
> {
> /*
> * If there are parallel threads are doing PTE changes on same range
> - * under non-exclusive lock(e.g., mmap_sem read-side) but defer TLB
> - * flush by batching, a thread has stable TLB entry can fail to flush
> - * the TLB by observing pte_none|!pte_dirty, for example so flush TLB
> - * forcefully if we detect parallel PTE batching threads.
> + * under non-exclusive lock (e.g., mmap_sem read-side) but defer TLB
> + * flush by batching, one thread may end up seeing inconsistent PTEs
> + * and result in having stale TLB entries. So flush TLB forcefully
> + * if we detect parallel PTE batching threads.
> + *
> + * However, some syscalls, e.g. munmap(), may free page tables, this
> + * needs force flush everything in the given range. Otherwise this
> + * may result in having stale TLB entries for some architectures,
> + * e.g. aarch64, that could specify flush what level TLB.
> */
> if (mm_tlb_flush_nested(tlb->mm)) {
> + /*
> + * The aarch64 yields better performance with fullmm by
> + * avoiding multiple CPUs spamming TLBI messages at the
> + * same time.
> + *
> + * On x86 non-fullmm doesn't yield significant difference
> + * against fullmm.
> + */
> + tlb->fullmm = 1;
> __tlb_reset_range(tlb);
> - __tlb_adjust_range(tlb, start, end - start);
> + tlb->freed_tables = 1;
> }
>
> tlb_flush_mmu(tlb);

Nick, Aneesh, can we now do this?

---

diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c b/arch/powerpc/mm/book3s64/radix_tlb.c
index 4d841369399f..8d28b83914cb 100644
--- a/arch/powerpc/mm/book3s64/radix_tlb.c
+++ b/arch/powerpc/mm/book3s64/radix_tlb.c
@@ -881,39 +881,6 @@ void radix__tlb_flush(struct mmu_gather *tlb)
*/
if (tlb->fullmm) {
__flush_all_mm(mm, true);
-#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLB_PAGE)
- } else if (mm_tlb_flush_nested(mm)) {
- /*
- * If there is a concurrent invalidation that is clearing ptes,
- * then it's possible this invalidation will miss one of those
- * cleared ptes and miss flushing the TLB. If this invalidate
- * returns before the other one flushes TLBs, that can result
- * in it returning while there are still valid TLBs inside the
- * range to be invalidated.
- *
- * See mm/memory.c:tlb_finish_mmu() for more details.
- *
- * The solution to this is ensure the entire range is always
- * flushed here. The problem for powerpc is that the flushes
- * are page size specific, so this "forced flush" would not
- * do the right thing if there are a mix of page sizes in
- * the range to be invalidated. So use __flush_tlb_range
- * which invalidates all possible page sizes in the range.
- *
- * PWC flush probably is not be required because the core code
- * shouldn't free page tables in this path, but accounting
- * for the possibility makes us a bit more robust.
- *
- * need_flush_all is an uncommon case because page table
- * teardown should be done with exclusive locks held (but
- * after locks are dropped another invalidate could come
- * in), it could be optimized further if necessary.
- */
- if (!tlb->need_flush_all)
- __radix__flush_tlb_range(mm, start, end, true);
- else
- radix__flush_all_mm(mm);
-#endif
} else if ( (psize = radix_get_mmu_psize(page_size)) == -1) {
if (!tlb->need_flush_all)
radix__flush_tlb_mm(mm);