linux-kernel - Re: [v2 PATCH] mm: mmu_gather: remove __tlb_reset

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Date:   Mon, 13 May 2019 17:38:04 +0100
From:   Will Deacon <will.deacon@....com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     jstancek@...hat.com, peterz@...radead.org, namit@...are.com,
        minchan@...nel.org, mgorman@...e.de, stable@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [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);