Re: [PATCH v5 06/19] selftests/mm: stop khugepaged during the MADV_COLLAPSE cases

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 9/8/26 8:50 PM, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
> 
> __madvise_collapse() turns THP off before each MADV_COLLAPSE, both to keep
> khugepaged out of the range and to prove MADV_COLLAPSE ignores the setting.
> It clears the global controls only, which is no longer enough.  A per-order
> control overrides them, and -s, which makes the cases fault in folios of
> one order, leaves that order's control at "always".  khugepaged then
> collapses the very range the case is working on, and the case fails on a
> collapse that was interfered with rather than refused.

Right. So I think the correct fix tag is b7f16963efe7 ("mm/khugepaged: 
run khugepaged for all orders"), because before this commit, khugepaged 
would not try to collapse this range since it only checked whether the 
PMD order was suitable for collapse.

> Clear the per-order controls too, setting them to "inherit" rather than
> "never": khugepaged honours the global never and stays out, while
> MADV_COLLAPSE on shmem still finds an order to build.
> 
> Fixes: 9f0704eae8a4 ("selftests/mm/khugepaged: enlighten for multi-size THP")
> Assisted-by: LLM
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> ---
>   tools/testing/selftests/mm/khugepaged.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index 398430c33872..e9bc8fe8a1f8 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -533,8 +533,8 @@ static bool is_anon(struct mem_ops *ops)
>   static void __madvise_collapse(const char *msg, char *p, int nr_hpages,
>   			       struct mem_ops *ops, bool expect)
>   {
> -	int ret;
>   	struct thp_settings settings = *thp_current_settings();
> +	int ret, i;
>   
>   	ksft_print_msg("%s...", msg);
>   
> @@ -547,9 +547,16 @@ static void __madvise_collapse(const char *msg, char *p, int nr_hpages,
>   	/*
>   	 * Prevent khugepaged interference and tests that MADV_COLLAPSE
>   	 * ignores /sys/kernel/mm/transparent_hugepage/enabled
> +	 *
> +	 * "inherit" rather than "never" so that MADV_COLLAPSE on shmem still
> +	 * finds an order to build.
>   	 */
>   	settings.thp_enabled = THP_NEVER;
>   	settings.shmem_enabled = SHMEM_NEVER;
> +	for (i = 0; i < NR_ORDERS; i++) {
> +		settings.hugepages[i].enabled = THP_INHERIT;
> +		settings.shmem_hugepages[i].enabled = SHMEM_INHERIT;
> +	}

This looks like a workaround to me. Shouldn't we fix this in shmem 
instead? Since MADV_COLLAPSE is supposed to ignore the THP setting, the 
fix should be something like this?

diff --git a/mm/shmem.c b/mm/shmem.c
index c92ed17dbc4a..e0c0901c34a4 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2050,7 +2050,7 @@ unsigned long shmem_allowable_huge_orders(struct 
inode *inode,
         global_orders = shmem_huge_global_enabled(inode, index, write_end,
                                                   shmem_huge_force, 
vma, vm_flags);
         /* Tmpfs huge pages allocation */
-       if (!vma || !vma_is_anon_shmem(vma))
+       if (!vma || !vma_is_anon_shmem(vma) || shmem_huge_force)
                 return global_orders;

         /*