linux-kernel - Re: [PATCH v2 2/5] mm: avoid unnecessary flush on change_huge

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Message-Id: <8BC74789-FF33-403F-B5D7-19034CAC7EE6@gmail.com>
Date:   Tue, 26 Oct 2021 12:06:40 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Nick Piggin <npiggin@...il.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v2 2/5] mm: avoid unnecessary flush on change_huge_pmd()



> On Oct 26, 2021, at 11:44 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 10/26/21 10:44 AM, Nadav Amit wrote:
>>> "If software on one logical processor writes to a page while software on
>>> another logical processor concurrently clears the R/W flag in the
>>> paging-structure entry that maps the page, execution on some processors may
>>> result in the entry’s dirty flag being set (due to the write on the first
>>> logical processor) and the entry’s R/W flag being clear (due to the update
>>> to the entry on the second logical processor). This will never occur on a
>>> processor that supports control-flow enforcement technology (CET)”
>>> 
>>> So I guess that this optimization can only be enabled when CET is enabled.
>>> 
>>> :(
>> I still wonder whether the SDM comment applies to present bit vs dirty
>> bit atomicity as well.
> 
> I think it's implicit.  From "4.8 ACCESSED AND DIRTY FLAGS":
> 
> 	"Whenever there is a write to a linear address, the processor
> 	 sets the dirty flag (if it is not already set) in the paging-
> 	 structure entry"
> 
> There can't be a "write to a linear address" without a Present=1 PTE.
> If it were a Dirty=1,Present=1 PTE, there's no race because there might
> not be a write to the PTE at all.
> 
> There's also this from the "4.10.4.3 Optional Invalidation" section:
> 
> 	"no TLB entry or paging-structure cache entry is created with
> 	 information from a paging-structure entry in which the P flag
> 	 is 0."
> 
> That means that we don't have to worry about the TLB doing something
> bonkers like caching a Dirty=1 bit from a Present=0 PTE.
> 
> Is that what you were worried about?

Thanks Dave, but no - that is not my concern.

To make it very clear - consider the following scenario, in which
a volatile pointer p is mapped using a certain PTE, which is RW
(i.e., *p is writable):

  CPU0				CPU1
  ----				----
  x = *p
  [ PTE cached in TLB; 
    PTE is not dirty ]
				clear_pte(PTE)
  *p = x
  [ needs to set dirty ]

Note that there is no TLB flush in this scenario. The question
is whether the write access to *p would succeed, setting the
dirty bit on the clear, non-present entry.

I was under the impression that the hardware AD-assist would
recheck the PTE atomically as it sets the dirty bit. But, as I
said, I am not sure anymore whether this is defined architecturally
(or at least would work in practice on all CPUs modulo the 
Knights Landing thingy).