linux-kernel - Re: [PATCH 6/6] x86: Fix stray A/D bit setting into non-present PTEs

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Date:	Thu, 30 Jun 2016 18:54:59 -0700
From:	Dave Hansen <dave@...1.net>
To:	Nadav Amit <nadav.amit@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	linux-mm@...ck.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, bp@...en8.de, ak@...ux.intel.com,
	mhocko@...e.com, dave.hansen@...ux.intel.com
Subject: Re: [PATCH 6/6] x86: Fix stray A/D bit setting into non-present PTEs

On 06/30/2016 06:50 PM, Nadav Amit wrote:
> Dave Hansen <dave@...1.net> wrote:
>> +pte_t ptep_clear_flush(struct vm_area_struct *vma, unsigned long address,
>> +		       pte_t *ptep)
>> +{
>> +	struct mm_struct *mm = vma->vm_mm;
>> +	pte_t pte;
>> +
>> +	pte = ptep_get_and_clear(mm, address, ptep);
>> +	if (pte_accessible(mm, pte)) {
>> +		flush_tlb_page(vma, address);
>> +		/*
>> +		 * Ensure that the compiler orders our set_pte()
>> +		 * after the flush_tlb_page() no matter what.
>> +		 */
>> +		barrier();
> 
> I don’t think such a barrier (after remote TLB flush) is needed.
> Eventually, if a remote flush takes place, you get csd_lock_wait() to be
> called, and then smp_rmb() is called (which is essentially a barrier()
> call on x86).

Andi really wanted to make sure this got in here.  He said there was a
bug that bit him really badly once where a function got reordered.
Granted, a call _should_ be sufficient to keep the compiler from
reordering things, but this makes double sure.