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From: Dave Hansen <dave.hansen@intel.com>
To: Nadav Amit <nadav.amit@gmail.com>, linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org, Nadav Amit <namit@vmware.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	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 2/5] mm: avoid unnecessary flush on change_huge_pmd()
Date: Tue, 26 Oct 2021 09:06:40 -0700	[thread overview]
Message-ID: <c415820a-aebb-265c-7f47-e048ee429102@intel.com> (raw)
In-Reply-To: <20211021122112.592634-3-namit@vmware.com>

On 10/21/21 5:21 AM, Nadav Amit wrote:
> The first TLB flush is only necessary to prevent the dirty bit (and with
> a lesser importance the access bit) from changing while the PTE is
> modified. However, this is not necessary as the x86 CPUs set the
> dirty-bit atomically with an additional check that the PTE is (still)
> present. One caveat is Intel's Knights Landing that has a bug and does
> not do so.

First, did I miss the check in this patch for X86_BUG_PTE_LEAK?  I don't
see it anywhere.

> -	 * pmdp_invalidate() is required to make sure we don't miss
> -	 * dirty/young flags set by hardware.

This got me thinking...  In here:

> https://lore.kernel.org/lkml/20160708001909.FB2443E2@viggo.jf.intel.com/

I wrote:

> These bits are truly "stray".  In the case of the Dirty bit, the
> thread associated with the stray set was *not* allowed to write to
> the page.  This means that we do not have to launder the bit(s); we
> can simply ignore them.

Is the goal of your proposed patch here to ensure that the dirty bit is
not set at *all*?  Or, is it to ensure that a dirty bit which we need to
*launder* is never set?


  parent reply	other threads:[~2021-10-26 16:10 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 [this message]
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
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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