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

Baolin Wang
2025-09-08T02:16:35Z

On 2025/9/5 22:11, 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@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>
> ---

Thanks for sorting this out. Make sense to me.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.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)) {