linux-kernel - Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with max_ptes

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Message-ID: <0cd87ad4-206c-4367-b7e8-9c1ccddc15c7@lucifer.local>
Date: Fri, 5 Sep 2025 16:30:09 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>, Zi Yan <ziy@...dia.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Usama Arif <usamaarif642@...il.com>
Subject: Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with
 max_ptes_none default

On Fri, Sep 05, 2025 at 04:11:37PM +0200, David Hildenbrand wrote:
> We added an early exit in thp_underused(), probably to avoid scanning
> pages when there is no chance for success.
>
> However, assume we have max_ptes_none = 511 (default).
>
> Nothing should stop us from freeing all pages part of a THP that

Freeing 'all pages which are part of a THP' rather?

> is completely zero (512) and khugepaged will for sure not try to

that is -> that are?

> instantiate a THP in that case (512 shared zeropages).

But if you write faulted they're not the zero page? And how are they shared? I
mean be being dumb here.

>
> This can just trivially happen if someone writes a single 0 byte into a
> PMD area, or of course, when data ends up being zero later.
>
> So let's remove that early exit.
>
> Do we want to CC stable? Hm, not sure. Probably not urgent.

Surely this is worth having?

>
> Note that, as default, the THP shrinker is active
> (/sys/kernel/mm/transparent_hugepage/shrink_underused = 1), and all
> THPs are added to the deferred split lists. However, with the
> max_ptes_none default we would never scan them. We would not do that. If

Nit but 'we would not do that' is kinda duplicative here :)

> that's not desirable, we should just disable the shrinker as default,
> also not adding all THPs to the deferred split lists.
>
> Easy to reproduce:
>
> 1) Allocate some THPs filled with 0s
>
> <prog.c>
>  #include <string.h>
>  #include <stdio.h>
>  #include <stdlib.h>
>  #include <unistd.h>
>  #include <sys/mman.h>
>
>  const size_t size = 1024*1024*1024;
>
>  int main(void)
>  {
>          size_t offs;
>          char *area;
>
>          area = mmap(0, size, PROT_READ | PROT_WRITE,
>                      MAP_ANON | MAP_PRIVATE, -1, 0);
>          if (area == MAP_FAILED) {
>                  printf("mmap failed\n");
>                  exit(-1);
>          }
>          madvise(area, size, MADV_HUGEPAGE);
>
>          for (offs = 0; offs < size; offs += getpagesize())
>                  area[offs] = 0;
>          pause();
>  }
> <\prog.c>
>
> 2) Trigger the shrinker
>
> E.g., memory pressure through memhog
>
> 3) Observe that THPs are not getting reclaimed
>
> $ cat /proc/`pgrep prog`/smaps_rollup
>
> Would list ~1GiB of AnonHugePages. With this fix, they would get
> reclaimed as expected.

Yikes!

>
> Fixes: dafff3f4c850 ("mm: split underused THPs")
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Nico Pache <npache@...hat.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Dev Jain <dev.jain@....com>
> Cc: Barry Song <baohua@...nel.org>
> Cc: Usama Arif <usamaarif642@...il.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
>  mm/huge_memory.c | 3 ---
>  1 file changed, 3 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 26cedfcd74189..aa3ed7a86435b 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4110,9 +4110,6 @@ static bool thp_underused(struct folio *folio)
>  	void *kaddr;
>  	int i;
>
> -	if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
> -		return false;
> -
>  	for (i = 0; i < folio_nr_pages(folio); i++) {
>  		kaddr = kmap_local_folio(folio, i * PAGE_SIZE);
>  		if (!memchr_inv(kaddr, 0, PAGE_SIZE)) {
> --
> 2.50.1
>