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Date: Fri, 2 Feb 2024 15:42:50 +0100
From: Michal Hocko <mhocko@suse.com>
To: Lance Yang <ioworker0@gmail.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
Subject: Re: [PATCH 1/1] mm/khugepaged: skip copying lazyfree pages on
 collapse
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On Fri 02-02-24 21:46:45, Lance Yang wrote:
> Here is a part from the man page explaining
> the MADV_FREE semantics:
> 
> The kernel can thus free thesepages, but the
> freeing could be delayed until memory pressure
> occurs. For each of the pages that has been
> marked to be freed but has not yet been freed,
> the free operation will be canceled if the caller
> writes into the page. If there is no subsequent
> write, the kernel can free the pages at any time.
> 
> IIUC, if there is no subsequent write, lazyfree
> pages will eventually be reclaimed.

If there is no memory pressure then this might not
ever happen. User cannot make any assumption about
their content once madvise call has been done. The
content has to be considered lost. Sure the userspace
might have means to tell those pages from zero pages
and recheck after the write but that is about it.

> khugepaged
> treats lazyfree pages the same as pte_none,
> avoiding copying them to the new huge page
> during collapse. It seems that lazyfree pages
> are reclaimed before khugepaged collapses them.
> This aligns with user expectations.
> 
> However, IMO, if the content of MADV_FREE pages
> remains valid during collapse, then khugepaged
> treating lazyfree pages the same as pte_none
> might not be suitable.

Why?

Unless I am missing something (which is possible of
course) I do not really see why dropping the content
of those pages and replacing them with a THP is any
difference from reclaiming those pages and then faulting
in a non-THP zero page.

Now, if khugepaged reused the original content of MADV_FREE
pages that would be a slightly different story. I can
see why users would expect zero pages to back madvised
area.
-- 
Michal Hocko
SUSE Labs