linux-kernel - Re: [PATCH 1/1] mm/khugepaged: skip copying lazyfree pages on collapse

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Date: Fri, 2 Feb 2024 11:06:40 +0100
From: Michal Hocko <mhocko@...e.com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, zokeefe@...gle.com, david@...hat.com,
	songmuchun@...edance.com, shy828301@...il.com, peterx@...hat.com,
	minchan@...nel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/khugepaged: skip copying lazyfree pages on
 collapse

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.
-- 
Michal Hocko
SUSE Labs