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

[Minchan Kim]

On Wed, Jun 24, 2020 at 01:00:14PM -0700, David Rientjes wrote:
> 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.

That's because of hint Jann provided in the past version.
https://lore.kernel.org/linux-api/CAG48ez27=pwm5m_N_988xT1huO7g7h6arTQL44zev6TD-h-7Tg@xxxxxxxxxxxxxx/

Thanks for the review, David.