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Subject: Re: [PATCH 2/2] mm: Remember young bit for page migrations
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <Yuv811AnT9aoBxbH@xz-m1.local>
Date:   Thu, 4 Aug 2022 10:16:50 -0700
Cc:     LKML <linux-kernel@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andi Kleen <andi.kleen@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Huang Ying <ying.huang@intel.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Vlastimil Babka <vbabka@suse.cz>,
        David Hildenbrand <david@redhat.com>,
        Minchan Kim <minchan@kernel.org>
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To:     Peter Xu <peterx@redhat.com>
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On Aug 4, 2022, at 10:07 AM, Peter Xu <peterx@redhat.com> wrote:

> On Wed, Aug 03, 2022 at 11:42:39PM -0700, Nadav Amit wrote:
>> On Aug 3, 2022, at 9:45 AM, Peter Xu <peterx@redhat.com> wrote:
> 
> For this series, I can think more of dropping the young bit for migration
> entry during these madvises (which should be relatively safe with the
> pgtable held, since I don't need to touch the page but just modify the swap
> entry within), but probably that's not really the major problem here, so
> not sure whether that matters a huge lot (e.g., for FREE we should really
> drop the whole entry?).

Sounds good to me. I just had a look whether the swap entry should be
checked and hit these additional cases, so I thought they are worthy of
consideration. But as they are already not handling some cases, which is
valid for madvise's hints, leaving it to later sounds fine.

Handling migration entries anyhow might not be worthy of the risk of
breaking something.