Re: [LTP] [PATCH v5] mmapstress06: Convert to new API

[Li Wang <li.wang@linux.dev>]

Wei Gao wrote:

> +
> +		mmapaddr = SAFE_MMAP(NULL, map_size, PROT_READ | PROT_WRITE,
> +				     MAP_ANONYMOUS | MAP_SHARED, -1, 0);
> +

> +		/*
> +		 * Disable Transparent Huge Pages (THP) for this mapping.
> +		 * Under tight cgroup memory limits (128 MB), allocating huge
> +		 * pages can easily trigger MemCG OOM-kills. Disabling THP forces
> +		 * standard 4 KB pages, ensuring stable swapping and avoiding
> +		 * premature OOM.
> +		 */
> +		if (madvise(mmapaddr, map_size, MADV_NOHUGEPAGE) < 0)
> +			tst_res(TWARN | TERRNO, "madvise(MADV_NOHUGEPAGE) failed");

The key point is that this is a MAP_ANONYMOUS | MAP_SHARED mapping.
Shared anonymous memory isn't backed by the normal anonymous-THP machinery,
it's backed internally by shmem/tmpfs.

THP behavior for shmem is governed by a separate knob:

  # cat /sys/kernel/mm/transparent_hugepage/shmem_enabled
  always within_size advise [never] deny force

basically all distro kernels this defaults to never.

So in the default configuration, no huge pages would ever be allocated for
this mapping, and the madvise(MADV_NOHUGEPAGE) is effectively a no-op.

-- 
Regards,
Li Wang

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