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SubjectRe: [v2 PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush
On Fri, May 10, 2019 at 07:26:54AM +0800, Yang Shi wrote:
> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> index 99740e1..469492d 100644
> --- a/mm/mmu_gather.c
> +++ b/mm/mmu_gather.c
> @@ -245,14 +245,39 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
> {
> /*
> * 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)) {
> - __tlb_reset_range(tlb);
> - __tlb_adjust_range(tlb, start, end - start);
> + if (mm_tlb_flush_nested(tlb->mm) && !tlb->fullmm) {
> + /*
> + * Since we can't tell what we actually should have
> + * flushed, flush everything in the given range.
> + */
> + tlb->freed_tables = 1;
> + tlb->cleared_ptes = 1;
> + tlb->cleared_pmds = 1;
> + tlb->cleared_puds = 1;
> + tlb->cleared_p4ds = 1;
> +
> + /*
> + * Some architectures, e.g. ARM, that have range invalidation
> + * and care about VM_EXEC for I-Cache invalidation, need force
> + * vma_exec set.
> + */
> + tlb->vma_exec = 1;
> +
> + /* Force vma_huge clear to guarantee safer flush */
> + tlb->vma_huge = 0;
> +
> + tlb->start = start;
> + tlb->end = end;
> }

Whilst I think this is correct, it would be interesting to see whether
or not it's actually faster than just nuking the whole mm, as I mentioned
before.

At least in terms of getting a short-term fix, I'd prefer the diff below
if it's not measurably worse.

Will

--->8

diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index 99740e1dd273..cc251422d307 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -251,8 +251,9 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
* forcefully if we detect parallel PTE batching threads.
*/
if (mm_tlb_flush_nested(tlb->mm)) {
+ tlb->fullmm = 1;
__tlb_reset_range(tlb);
- __tlb_adjust_range(tlb, start, end - start);
+ tlb->freed_tables = 1;
}

tlb_flush_mmu(tlb);
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