Re: [PATCH v8 3/4] mm/madvise: introduce process_madvise() syscall: an external memory hinting API

[David Rientjes]

On Mon, 22 Jun 2020, Minchan Kim wrote:

> diff --git a/mm/madvise.c b/mm/madvise.c
> index 551ed816eefe..23abca3f93fa 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -17,6 +17,7 @@
>  #include <linux/falloc.h>
>  #include <linux/fadvise.h>
>  #include <linux/sched.h>
> +#include <linux/sched/mm.h>
>  #include <linux/ksm.h>
>  #include <linux/fs.h>
>  #include <linux/file.h>
> @@ -995,6 +996,18 @@ madvise_behavior_valid(int behavior)
>  	}
>  }
>  
> +static bool
> +process_madvise_behavior_valid(int behavior)
> +{
> +	switch (behavior) {
> +	case MADV_COLD:
> +	case MADV_PAGEOUT:
> +		return true;
> +	default:
> +		return false;
> +	}
> +}
> +
>  /*
>   * The madvise(2) system call.
>   *
> @@ -1042,6 +1055,11 @@ madvise_behavior_valid(int behavior)
>   *  MADV_DONTDUMP - the application wants to prevent pages in the given range
>   *		from being included in its core dump.
>   *  MADV_DODUMP - cancel MADV_DONTDUMP: no longer exclude from core dump.
> + *  MADV_COLD - the application is not expected to use this memory soon,
> + *		deactivate pages in this range so that they can be reclaimed
> + *		easily if memory pressure hanppens.
> + *  MADV_PAGEOUT - the application is not expected to use this memory soon,
> + *		page out the pages in this range immediately.
>   *
>   * return values:
>   *  zero    - success
> @@ -1176,3 +1194,106 @@ SYSCALL_DEFINE3(madvise, unsigned long, start, size_t, len_in, int, behavior)
>  {
>  	return do_madvise(current, current->mm, start, len_in, behavior);
>  }
> +
> +static int process_madvise_vec(struct task_struct *target_task,
> +		struct mm_struct *mm, struct iov_iter *iter, int behavior)
> +{
> +	struct iovec iovec;
> +	int ret = 0;
> +
> +	while (iov_iter_count(iter)) {
> +		iovec = iov_iter_iovec(iter);
> +		ret = do_madvise(target_task, mm, (unsigned long)iovec.iov_base,
> +					iovec.iov_len, behavior);
> +		if (ret < 0)
> +			break;
> +		iov_iter_advance(iter, iovec.iov_len);
> +	}
> +
> +	return ret;
> +}
> +
> +static ssize_t do_process_madvise(int pidfd, struct iov_iter *iter,
> +				int behavior, unsigned int flags)
> +{
> +	ssize_t ret;
> +	struct pid *pid;
> +	struct task_struct *task;
> +	struct mm_struct *mm;
> +	size_t total_len = iov_iter_count(iter);
> +
> +	if (flags != 0)
> +		return -EINVAL;
> +
> +	pid = pidfd_get_pid(pidfd);
> +	if (IS_ERR(pid))
> +		return PTR_ERR(pid);
> +
> +	task = get_pid_task(pid, PIDTYPE_PID);
> +	if (!task) {
> +		ret = -ESRCH;
> +		goto put_pid;
> +	}
> +
> +	if (task->mm != current->mm &&
> +			!process_madvise_behavior_valid(behavior)) {
> +		ret = -EINVAL;
> +		goto release_task;
> +	}
> +
> +	mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS);
> +	if (IS_ERR_OR_NULL(mm)) {
> +		ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
> +		goto release_task;
> +	}
> 

mm is always task->mm right?  I'm wondering if it would be better to find 
the mm directly in process_madvise_vec() rather than passing it into the 
function.  I'm not sure why we'd pass both task and mm here.

+
> +	ret = process_madvise_vec(task, mm, iter, behavior);
> +	if (ret >= 0)
> +		ret = total_len - iov_iter_count(iter);
> +
> +	mmput(mm);
> +release_task:
> +	put_task_struct(task);
> +put_pid:
> +	put_pid(pid);
> +	return ret;
> +}
> +
> +SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
> +		unsigned long, vlen, int, behavior, unsigned int, flags)

I love the idea of adding the flags parameter here and I can think of an 
immediate use case for MADV_HUGEPAGE, which is overloaded.

Today, MADV_HUGEPAGE controls enablement depending on system config and 
controls defrag behavior based on system config.  It also cannot be opted 
out of without setting MADV_NOHUGEPAGE :)

I was thinking of a flag that users could use to trigger an immediate 
collapse in process context regardless of the system config.

So I'm a big advocate of this flags parameter and consider it an absolute 
must for the API.

Acked-by: David Rientjes <rientjes@xxxxxxxxxx>