Message-ID: <7B0B1E09-5083-449F-851D-FD63D32D2B3D@nvidia.com>
Date: Fri, 05 Sep 2025 10:37:00 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.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 5 Sep 2025, at 10:11, 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
> is completely zero (512) and khugepaged will for sure not try to
> instantiate a THP in that case (512 shared zeropages).
>
> 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.
>
> 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
> 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.
>
> 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>
> ---
>  mm/huge_memory.c | 3 ---
>  1 file changed, 3 deletions(-)
>
LGTM. Acked-by: Zi Yan <ziy@...dia.com>

I also notice that thp_underused() checks num_zero_pages directly
against khugepaged_max_ptes_none. This means mTHPs will never be regarded
as underused. A similar issue you are discussing in Nico’s khugepaged
mTHP support. Maybe checks against these khugepaged_max* variables
should be calculated based on nr_pages of a large folio, like
making these variables a ratio in other discussion.

Best Regards,
Yan, Zi

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