Re: [PATCH v2 0/5] mm/mprotect: avoid unnecessary TLB flushes

From: Nadav Amit <nadav.amit@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Linux-MM <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Andi Kleen <ak@linux.intel.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Xu <peterx@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
	Nick Piggin <npiggin@gmail.com>,
	x86@kernel.org
Subject: Re: [PATCH v2 0/5] mm/mprotect: avoid unnecessary TLB flushes
Date: Mon, 25 Oct 2021 09:42:19 -0700	[thread overview]
Message-ID: <B197B99D-14BA-4ED4-A5ED-91A48A403735@gmail.com> (raw)
In-Reply-To: <YXaL2Cq7XI/gQNCZ@hirez.programming.kicks-ass.net>

> On Oct 25, 2021, at 3:50 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> On Thu, Oct 21, 2021 at 08:04:50PM -0700, Andrew Morton wrote:
>> On Thu, 21 Oct 2021 05:21:07 -0700 Nadav Amit <nadav.amit@gmail.com> wrote:
>> 
>>> This patch-set is intended to remove unnecessary TLB flushes. It is
>>> based on feedback from v1 and several bugs I found in v1 myself.
>>> 
>>> Basically, there are 3 optimizations in this patch-set:
>>> 1. Avoiding TLB flushes on change_huge_pmd() that are only needed to
>>>   prevent the A/D bits from changing.
>>> 2. Use TLB batching infrastructure to batch flushes across VMAs and
>>>   do better/fewer flushes.
>>> 3. Avoid TLB flushes on permission demotion.
>>> 
>>> Andrea asked for the aforementioned (2) to come after (3), but this
>>> is not simple (specifically since change_prot_numa() needs the number
>>> of pages affected).
>> 
>> [1/5] appears to be a significant fix which should probably be
>> backported into -stable kernels.  If you agree with this then I suggest
>> it be prepared as a standalone patch, separate from the other four
>> patches.  With a cc:stable.
> 
> I am confused, 1/5 doesn't actually do *anything*. I also cannot find
> any further usage of the introduced X86_BUG_PTE_LEAK.
> 
> I'm thinking patch #2 means to have something like:
> 
> 	if (cpu_feature_enabled(X86_BUG_PTE_LEAK))
> 		flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> 
> In the newly minted: pmdp_invalidate_ad(), but alas, nothing there.

This change was only intended for pmdp_invalidate_ad() but somehow
got lost. I will add it there.

I eventually did not add the optimization to avoid TLB flushes on
(!dirty|write)->!write so I did not use it for the first case that
you mentioned. I am too afraid, although I think this is correct.
Perhaps I will add it as a separate patch.