linux-kernel - Re: [PATCH 1/1] mm: avoid processing mlocked THPs in deferred split shrinker

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <5a1429ad-3900-404a-bdca-f25623ce603a@redhat.com>
Date: Mon, 8 Sep 2025 09:38:18 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org
Cc: Liam.Howlett@...cle.com, baohua@...nel.org,
 baolin.wang@...ux.alibaba.com, dev.jain@....com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 lorenzo.stoakes@...cle.com, npache@...hat.com, ryan.roberts@....com,
 usamaarif642@...il.com, ziy@...dia.com
Subject: Re: [PATCH 1/1] mm: avoid processing mlocked THPs in deferred split
 shrinker

On 08.09.25 06:07, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>

Subject should likely be more specific:

mm: skip mlocked THPs that are underused early in deferred_split_scan()

> 
> When a new THP is faulted in or collapsed, it is unconditionally added to
> the deferred split queue. If this THP is subsequently mlocked, it remains
> on the queue but is removed from the LRU and marked unevictable.
> 
> During memory reclaim, deferred_split_scan() will still pick up this large
> folio. Because it's not partially mapped, it will proceed to call
> thp_underused() and then attempt to split_folio() to free all zero-filled
> subpages.
> 
> This is a pointless waste of CPU cycles. The folio is mlocked and
> unevictable, so any attempt to reclaim memory from it via splitting is
> doomed to fail.

I think the whole description is a bit misleading: we're not reclaiming
memory from fully-mapped THPs even when they are underused, because it
could violate mlock() semantics where we don't want a page fault+memory 
allocation on next access.

So something like the following might be clearer.

"When we stumble over a fully-mapped THP in the deferred shrinker, it 
does not make sense trying to detect whether it is underused, because 
try_to_map_unused_to_zeropage(), called while splitting the folio, will 
not actually replace any zero-ed pages by the shared zeropage.

Splitting the folio in that case does not make any sense, so let's not 
even scan if the folio is underused.
"



If I run my reproducer from [1] and mlock() the pages just after 
allocating them, then I essentially get

AnonHugePages:   1048576 kB

converted to

Anonymous:       1048580 kB

Which makes sense (no memory optimized out) as discussed above.


[1] https://lkml.kernel.org/r/20250905141137.3529867-1-david@redhat.com

> 
> So, let's add an early folio_test_mlocked() check to skip this case.
> 
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---
>   mm/huge_memory.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 77f0c3417973..d2e84015d6b4 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4183,6 +4183,9 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>   		bool underused = false;
>   
>   		if (!folio_test_partially_mapped(folio)) {
> +			/* An mlocked folio is not a candidate for the shrinker. */

/*
  * See try_to_map_unused_to_zeropage(): we cannot optimize zero-filled
  * pages after splitting an mlocked folio.
  */

> +			if (folio_test_mlocked(folio))
> +				goto next;
>   			underused = thp_underused(folio);
>   			if (!underused)
>   				goto next;


-- 
Cheers

David / dhildenb