linux-kernel - Re: [PATCH v3 5/7] mm: Remember young/dirty bit for page migrations

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Date:   Thu, 18 Aug 2022 09:39:19 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Nadav Amit <nadav.amit@...il.com>, Peter Xu <peterx@...hat.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Minchan Kim <minchan@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <andi.kleen@...el.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v3 5/7] mm: Remember young/dirty bit for page migrations

On 8/15/22 14:03, Nadav Amit wrote:
>>
>> At least on x86, the hardware is not supposed to do so. The only case I
>> remember (and sometimes misremembers) is with KNL erratum, which perhaps
>> needs to be considered:
>>
>> https://lore.kernel.org/all/20160708001911.9A3FD2B6@viggo.jf.intel.com/
> I keep not remembering this erratum correctly. IIRC, the erratum says that
> the access/dirty might be set, but it does not mean that a write is possible
> after the PTE is cleared (i.e., the dirty/access might be set on the
> non-present PTE, but the access itself would fail). So it is not an issue in
> this case - losing A/D would not impact correctness since the access should
> fail.
> 
> Dave Hansen hates when I get confused with this one, but I cc him if he
> wants to confirm.

Right.

The issue is strictly with the page walker setting Accessed/Dirty in a
racy way.  The TLB still has accurate contents at all times.