Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with max_ptes_none default

[Usama Arif <usamaarif642@gmail.com>]

On 05/09/2025 15:11, David Hildenbrand wrote:
> We added an early exit in thp_underused(), probably to avoid scanning
> pages when there is no chance for success.
> 

Yes, that was the main reason.

> 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).

Agree with this point for this patch.

> 
> 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.
> 

The reason I did this is for the case if you change max_ptes_none after the THP is added
to deferred split list but *before* memory pressure, i.e. before the shrinker runs,
so that its considered for splitting.

> 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@linux-foundation.org>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> Cc: Nico Pache <npache@redhat.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Barry Song <baohua@kernel.org>
> Cc: Usama Arif <usamaarif642@gmail.com>
> Signed-off-by: David Hildenbrand <david@redhat.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;
> -

I do agree with your usecase, but I am really worried about the amount of
work and cpu time the THP shrinker will consume when max_ptes_none is 511
(I dont have any numbers to back up my worry :)), and its less likely that
we will have these completely zeroed out THPs (again no numbers to back up
this statement). We have the huge_zero_folio as well which is installed on read.

>  	for (i = 0; i < folio_nr_pages(folio); i++) {
>  		kaddr = kmap_local_folio(folio, i * PAGE_SIZE);
>  		if (!memchr_inv(kaddr, 0, PAGE_SIZE)) {