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From: Lance Yang <ioworker0@gmail.com>
Date: Fri, 2 Feb 2024 19:18:31 +0800
Message-ID: <CAK1f24=7sy_Bczpt5YeDbkhfriYUc1=zreSFdGCxfF3R0D6sRQ@mail.gmail.com>
Subject: Re: [PATCH 1/1] mm/khugepaged: skip copying lazyfree pages on collapse
To: Michal Hocko <mhocko@suse.com>
Cc: akpm@linux-foundation.org, zokeefe@google.com, david@redhat.com, 
	songmuchun@bytedance.com, shy828301@gmail.com, peterx@redhat.com, 
	minchan@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org
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On Fri, Feb 2, 2024 at 6:06=E2=80=AFPM Michal Hocko <mhocko@suse.com> wrote=
:
>
> On Thu 01-02-24 20:52:26, Lance Yang wrote:
> > The collapsing behavior of khugepaged with pages
> > marked using MADV_FREE might cause confusion
> > among users.
> >
> > For instance, allocate a 2MB chunk using mmap and
> > later release it by MADV_FREE. Khugepaged will not
> > collapse this chunk. From the user's perspective,
> > it treats lazyfree pages as pte_none. However,
> > for some pages marked as lazyfree with MADV_FREE,
> > khugepaged might collapse this chunk and copy
> > these pages to a new huge page. This inconsistency
> > in behavior could be confusing for users.
>
> Is that any more confusing than collapsing pte_none
> pages?
>
> TBH I do not really see why this is a problem. MADV_FREE
> are correctly recognized same as pte_none so the user
> defined trashold applies.

Sorry, I might not have expressed it clearly.
MADV_FREE is correctly recognized, just like
pte_none. This is also reasonable and not an issue.

IMO, since it's treated the same as pte_none,
perhaps lazyfree pages shouldn't be copied to
the new huge page.

Thanks,
Lance
> --
> Michal Hocko
> SUSE Labs