From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Nadav Amit <namit@vmware.com>
Cc: Andy Lutomirski <luto@kernel.org>, Linux-MM <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Dave Hansen <dave.hansen@linux.intel.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 ML <x86@kernel.org>,
Andy Lutomirski <luto@amacapital.net>
Subject: Re: [RFC 03/20] mm/mprotect: do not flush on permission promotion
Date: Mon, 1 Feb 2021 15:38:42 +0000 [thread overview]
Message-ID: <c9f963fa-076e-2504-c001-da69315d3732@citrix.com> (raw)
In-Reply-To: <B24BAD2A-6641-42E3-9098-3A41AA39D417@vmware.com>
On 01/02/2021 05:58, Nadav Amit wrote:
>> On Jan 31, 2021, at 4:10 AM, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>>
>> On 31/01/2021 01:07, Andy Lutomirski wrote:
>>> Adding Andrew Cooper, who has a distressingly extensive understanding
>>> of the x86 PTE magic.
>> Pretty sure it is all learning things the hard way...
>>
>>> On Sat, Jan 30, 2021 at 4:16 PM Nadav Amit <nadav.amit@gmail.com> wrote:
>>>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>>>> index 632d5a677d3f..b7473d2c9a1f 100644
>>>> --- a/mm/mprotect.c
>>>> +++ b/mm/mprotect.c
>>>> @@ -139,7 +139,8 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
>>>> ptent = pte_mkwrite(ptent);
>>>> }
>>>> ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent);
>>>> - tlb_flush_pte_range(tlb, addr, PAGE_SIZE);
>>>> + if (pte_may_need_flush(oldpte, ptent))
>>>> + tlb_flush_pte_range(tlb, addr, PAGE_SIZE);
>> You're choosing to avoid the flush, based on A/D bits read ahead of the
>> actual modification of the PTE.
>>
>> In this example, another thread can write into the range (sets A and D),
>> and get a suitable TLB entry which goes unflushed while the rest of the
>> kernel thinks the memory is write-protected and clean.
>>
>> The only safe way to do this is to use XCHG/etc to modify the PTE, and
>> base flush calculations on the results. Atomic operations are ordered
>> with A/D updates from pagewalks on other CPUs, even on AMD where A
>> updates are explicitly not ordered with regular memory reads, for
>> performance reasons.
> Thanks Andrew for the feedback, but I think the patch does it exactly in
> this safe manner that you describe (at least on native x86, but I see a
> similar path elsewhere as well):
>
> oldpte = ptep_modify_prot_start()
> -> __ptep_modify_prot_start()
> -> ptep_get_and_clear
> -> native_ptep_get_and_clear()
> -> xchg()
>
> Note that the xchg() will clear the PTE (i.e., making it non-present), and
> no further updates of A/D are possible until ptep_modify_prot_commit() is
> called.
>
> On non-SMP setups this is not atomic (no xchg), but since we hold the lock,
> we should be safe.
>
> I guess you are right and a pte_may_need_flush() deserves a comment to
> clarify that oldpte must be obtained by an atomic operation to ensure no A/D
> bits are lost (as you say).
>
> Yet, I do not see a correctness problem. Am I missing something?
No(ish) - I failed to spot that path.
But native_ptep_get_and_clear() is broken on !SMP builds. It needs to
be an XCHG even in that case, to spot A/D updates from prefetch or
shared-virtual-memory DMA.
~Andrew
next prev parent reply other threads:[~2021-02-01 15:40 UTC|newest]
Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-31 0:11 [RFC 00/20] TLB batching consolidation and enhancements Nadav Amit
2021-01-31 0:11 ` [RFC 01/20] mm/tlb: fix fullmm semantics Nadav Amit
2021-01-31 1:02 ` Andy Lutomirski
2021-01-31 1:19 ` Nadav Amit
2021-01-31 2:57 ` Andy Lutomirski
2021-02-01 7:30 ` Nadav Amit
2021-02-01 11:36 ` Peter Zijlstra
2021-02-02 9:32 ` Nadav Amit
2021-02-02 11:00 ` Peter Zijlstra
2021-02-02 21:35 ` Nadav Amit
2021-02-03 9:44 ` Will Deacon
2021-02-04 3:20 ` Nadav Amit
2021-01-31 0:11 ` [RFC 02/20] mm/mprotect: use mmu_gather Nadav Amit
2021-01-31 0:11 ` [RFC 03/20] mm/mprotect: do not flush on permission promotion Nadav Amit
2021-01-31 1:07 ` Andy Lutomirski
2021-01-31 1:17 ` Nadav Amit
2021-01-31 2:59 ` Andy Lutomirski
[not found] ` <7a6de15a-a570-31f2-14d6-a8010296e694@citrix.com>
2021-02-01 5:58 ` Nadav Amit
2021-02-01 15:38 ` Andrew Cooper [this message]
2021-01-31 0:11 ` [RFC 04/20] mm/mapping_dirty_helpers: use mmu_gather Nadav Amit
2021-01-31 0:11 ` [RFC 05/20] mm/tlb: move BATCHED_UNMAP_TLB_FLUSH to tlb.h Nadav Amit
2021-01-31 0:11 ` [RFC 06/20] fs/task_mmu: use mmu_gather interface of clear-soft-dirty Nadav Amit
2021-01-31 0:11 ` [RFC 07/20] mm: move x86 tlb_gen to generic code Nadav Amit
2021-01-31 18:26 ` Andy Lutomirski
2021-01-31 0:11 ` [RFC 08/20] mm: store completed TLB generation Nadav Amit
2021-01-31 20:32 ` Andy Lutomirski
2021-02-01 7:28 ` Nadav Amit
2021-02-01 16:53 ` Andy Lutomirski
2021-02-01 11:52 ` Peter Zijlstra
2021-01-31 0:11 ` [RFC 09/20] mm: create pte/pmd_tlb_flush_pending() Nadav Amit
2021-01-31 0:11 ` [RFC 10/20] mm: add pte_to_page() Nadav Amit
2021-01-31 0:11 ` [RFC 11/20] mm/tlb: remove arch-specific tlb_start/end_vma() Nadav Amit
2021-02-01 12:09 ` Peter Zijlstra
2021-02-02 6:41 ` Nicholas Piggin
2021-02-02 7:20 ` Nadav Amit
2021-02-02 9:31 ` Peter Zijlstra
2021-02-02 9:54 ` Nadav Amit
2021-02-02 11:04 ` Peter Zijlstra
2021-01-31 0:11 ` [RFC 12/20] mm/tlb: save the VMA that is flushed during tlb_start_vma() Nadav Amit
2021-02-01 12:28 ` Peter Zijlstra
2021-01-31 0:11 ` [RFC 13/20] mm/tlb: introduce tlb_start_ptes() and tlb_end_ptes() Nadav Amit
[not found] ` <YBaBcc2jEGaxuxH0@fedora.tometzki.de>
2021-02-01 7:29 ` Nadav Amit
2021-02-01 13:19 ` Peter Zijlstra
2021-02-01 23:00 ` Nadav Amit
2021-01-31 0:11 ` [RFC 14/20] mm: move inc/dec_tlb_flush_pending() to mmu_gather.c Nadav Amit
2021-01-31 0:11 ` [RFC 15/20] mm: detect deferred TLB flushes in vma granularity Nadav Amit
2021-02-01 22:04 ` Nadav Amit
2021-02-02 0:14 ` Andy Lutomirski
2021-02-02 20:51 ` Nadav Amit
2021-02-04 4:35 ` Andy Lutomirski
2021-01-31 0:11 ` [RFC 16/20] mm/tlb: per-page table generation tracking Nadav Amit
2021-01-31 0:11 ` [RFC 17/20] mm/tlb: updated completed deferred TLB flush conditionally Nadav Amit
2021-01-31 0:11 ` [RFC 18/20] mm: make mm_cpumask() volatile Nadav Amit
2021-01-31 0:11 ` [RFC 19/20] lib/cpumask: introduce cpumask_atomic_or() Nadav Amit
2021-01-31 0:11 ` [RFC 20/20] mm/rmap: avoid potential races Nadav Amit
2021-08-23 8:05 ` Huang, Ying
2021-08-23 15:50 ` Nadav Amit
2021-08-24 0:36 ` Huang, Ying
2021-01-31 0:39 ` [RFC 00/20] TLB batching consolidation and enhancements Andy Lutomirski
2021-01-31 1:08 ` Nadav Amit
2021-01-31 3:30 ` Nicholas Piggin
2021-01-31 7:57 ` Nadav Amit
2021-01-31 8:14 ` Nadav Amit
2021-02-01 12:44 ` Peter Zijlstra
2021-02-02 7:14 ` Nicholas Piggin
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=c9f963fa-076e-2504-c001-da69315d3732@citrix.com \
--to=andrew.cooper3@citrix.com \
--cc=aarcange@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=dave.hansen@linux.intel.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=luto@amacapital.net \
--cc=luto@kernel.org \
--cc=namit@vmware.com \
--cc=npiggin@gmail.com \
--cc=peterz@infradead.org \
--cc=tglx@linutronix.de \
--cc=will@kernel.org \
--cc=x86@kernel.org \
--cc=yuzhao@google.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).