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Subject: Re: [PATCH v2 0/5] mm/mprotect: avoid unnecessary TLB flushes
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20211021200450.b13499c379a27dbfefe9f5e3@linux-foundation.org>
Date:   Fri, 22 Oct 2021 14:58:39 -0700
Cc:     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>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Nick Piggin <npiggin@gmail.com>, x86@kernel.org
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Message-Id: <9E576E79-E4FD-4CB9-8BE5-142230C12E63@gmail.com>
References: <20211021122112.592634-1-namit@vmware.com>
 <20211021200450.b13499c379a27dbfefe9f5e3@linux-foundation.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Precedence: bulk


> On Oct 21, 2021, at 8:04 PM, Andrew Morton <akpm@linux-foundation.org> =
wrote:
>=20
> On Thu, 21 Oct 2021 05:21:07 -0700 Nadav Amit <nadav.amit@gmail.com> =
wrote:
>=20
>> 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.
>>=20
>> 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.
>>=20
>> 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).
>=20
> [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.


There is no functionality bug in the kernel. The Knights Landing bug
was circumvented eventually by changing the swap entry structure so
the access/dirty bits would not overlap with the swap entry data.

>=20
> And the remaining patches are a performance optimization.  Has any
> attempt been made to quantify the benefits?

I included some data before [1]. In general the cost that is saved
is the cost of a TLB flush/shootdown.

I will modify my benchmark to test huge-pages (which were not
included in the previous patch-set) and send results later. I would
also try nodejs to see if there is a significant enough benefit.
Nodejs crashed before (hence the 3rd patch added here), as it
exec-protects/unprotects pages - I will see if the benefit shows in
the benchmarks.

[ The motivation behind the patches is to later introduce userfaultfd
writeprotectv interface, and for my use-case that is under=20
development this proved to improve performance considerably. ]


[1] =
https://lore.kernel.org/linux-mm/DA49DBBB-FFEE-4ACC-BB6C-364D07533C5E@vmwa=
re.com/=