Re: [LTP] memcg/memcontrol04: Fix race in pagecache allocation measurement

[linuxtestproject.agent@gmail.com]

Hi Pavithra,

On Mon, 25 May 2026, Pavithra wrote:
> memcg/memcontrol04: Fix race in pagecache allocation measurement

> +static unsigned int num_children_spawned;

> +enum checkpoints {
> -	CHILD_IDLE
> +	CHILD_IDLE,
> +	TEST_DONE,
>  };

> +	if (num_children_spawned > 0) {
> +		TST_CHECKPOINT_WAKE2(TEST_DONE, num_children_spawned);
> +		tst_reap_children();
> +		num_children_spawned = 0;
> +	}

Correct. Waking all pagecache children before tst_reap_children() and
resetting the counter is safe since cleanup_sub_groups() is idempotent
(called both from test_memcg_low() and cleanup()).

> -		tst_reap_children();
> +		SAFE_WAITPID(pid, NULL, 0);

Good catch. The old tst_reap_children() would have reaped the still-
waiting pagecache children prematurely.

> +	num_children_spawned++;
> +	TST_CHECKPOINT_WAIT(CHILD_IDLE);

Sequential fork+wait pairs mean CHILD_IDLE is always at zero before
the next child is forked, so there's no ambiguity in who signals it.

> +	SAFE_FSYNC(fd);
> +
> +	TST_CHECKPOINT_WAKE(CHILD_IDLE);
> +	TST_CHECKPOINT_WAIT(TEST_DONE);

fsync before signalling the parent ensures the pagecache is durably
accounted before memory.current is read.

> +	num_children_spawned = 0;
>  	fd = SAFE_OPEN(TMPDIR"/tmpfile", O_RDWR | O_CREAT, 0600);

Explicit reset at the start of each test_all invocation is the right
place; handles repeated runs correctly.

Reviewed-by: LTP AI Reviewer <ltp-ai@noreply.github.com>

---
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