From: Nadav Amit <nadav.amit@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>
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
Subject: Re: [PATCH v2 0/5] mm/mprotect: avoid unnecessary TLB flushes
Date: Fri, 22 Oct 2021 14:58:39 -0700 [thread overview]
Message-ID: <9E576E79-E4FD-4CB9-8BE5-142230C12E63@gmail.com> (raw)
In-Reply-To: <20211021200450.b13499c379a27dbfefe9f5e3@linux-foundation.org>
> On Oct 21, 2021, at 8:04 PM, Andrew Morton <akpm@linux-foundation.org> 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.
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.
>
> 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
development this proved to improve performance considerably. ]
[1] https://lore.kernel.org/linux-mm/DA49DBBB-FFEE-4ACC-BB6C-364D07533C5E@vmware.com/
next prev parent reply other threads:[~2021-10-22 21:58 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-21 12:21 [PATCH v2 0/5] mm/mprotect: avoid unnecessary TLB flushes Nadav Amit
2021-10-21 12:21 ` [PATCH v2 1/5] x86: Detection of Knights Landing A/D leak Nadav Amit
2021-10-26 15:54 ` Dave Hansen
2021-10-26 15:57 ` Nadav Amit
2021-10-21 12:21 ` [PATCH v2 2/5] mm: avoid unnecessary flush on change_huge_pmd() Nadav Amit
2021-10-25 10:52 ` Peter Zijlstra
2021-10-25 16:29 ` Nadav Amit
2021-10-26 16:06 ` Dave Hansen
2021-10-26 16:47 ` Nadav Amit
2021-10-26 16:53 ` Nadav Amit
2021-10-26 17:44 ` Nadav Amit
2021-10-26 18:44 ` Dave Hansen
2021-10-26 19:06 ` Nadav Amit
2021-10-26 19:40 ` Dave Hansen
2021-10-26 20:07 ` Nadav Amit
2021-10-26 20:47 ` Dave Hansen
2021-10-21 12:21 ` [PATCH v2 3/5] x86/mm: check exec permissions on fault Nadav Amit
2021-10-25 10:59 ` Peter Zijlstra
2021-10-25 11:13 ` Andrew Cooper
2021-10-25 14:23 ` Dave Hansen
2021-10-25 14:20 ` Dave Hansen
2021-10-25 16:19 ` Nadav Amit
2021-10-25 17:45 ` Dave Hansen
2021-10-25 17:51 ` Nadav Amit
2021-10-25 18:00 ` Dave Hansen
2021-10-21 12:21 ` [PATCH v2 4/5] mm/mprotect: use mmu_gather Nadav Amit
2021-10-21 12:21 ` [PATCH v2 5/5] mm/mprotect: do not flush on permission promotion Nadav Amit
2021-10-25 11:12 ` Peter Zijlstra
2021-10-25 16:27 ` Nadav Amit
2021-10-22 3:04 ` [PATCH v2 0/5] mm/mprotect: avoid unnecessary TLB flushes Andrew Morton
2021-10-22 21:58 ` Nadav Amit [this message]
2021-10-26 16:09 ` Dave Hansen
2021-10-25 10:50 ` Peter Zijlstra
2021-10-25 16:42 ` Nadav Amit
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