Re: [PATCH v4 1/8] mm: pass task to do_madvise

[Alexander Duyck]

On Wed, 2020-02-12 at 15:39 -0800, Minchan Kim wrote:
> In upcoming patches, do_madvise will be called from external process
> context so it shouldn't asssume "current" is always hinted process's
> task_struct. Thus, let's get the mm_struct from vma->vm_mm, not
> current because vma is always hinted process's one. And let's pass
> *current* as new task argument of do_madvise so it shouldn't change
> existing behavior.
> 
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> ---
>  fs/io_uring.c      |  2 +-
>  include/linux/mm.h |  3 ++-
>  mm/madvise.c       | 37 ++++++++++++++++++++-----------------
>  3 files changed, 23 insertions(+), 19 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 63beda9bafc5..6307206b970f 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2736,7 +2736,7 @@ static int io_madvise(struct io_kiocb *req, struct io_kiocb **nxt,
>  	if (force_nonblock)
>  		return -EAGAIN;
>  
> -	ret = do_madvise(ma->addr, ma->len, ma->advice);
> +	ret = do_madvise(current, ma->addr, ma->len, ma->advice);
>  	if (ret < 0)
>  		req_set_fail_links(req);
>  	io_cqring_add_event(req, ret);
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 52269e56c514..8cb41131ec96 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2323,7 +2323,8 @@ extern int __do_munmap(struct mm_struct *, unsigned long, size_t,
>  		       struct list_head *uf, bool downgrade);
>  extern int do_munmap(struct mm_struct *, unsigned long, size_t,
>  		     struct list_head *uf);
> -extern int do_madvise(unsigned long start, size_t len_in, int behavior);
> +extern int do_madvise(struct task_struct *task, unsigned long start,
> +			size_t len_in, int behavior);
>  
>  static inline unsigned long
>  do_mmap_pgoff(struct file *file, unsigned long addr,
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 43b47d3fae02..ab4011ba2d9e 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -256,6 +256,7 @@ static long madvise_willneed(struct vm_area_struct *vma,
>  {
>  	struct file *file = vma->vm_file;
>  	loff_t offset;
> +	struct mm_struct *mm = vma->vm_mm;
>  
>  	*prev = vma;
>  #ifdef CONFIG_SWAP

I would probably move the declaration of the mm variable to the top just
so you don't have the large "offset" valley between the two long variable
declarations.

> @@ -288,12 +289,12 @@ static long madvise_willneed(struct vm_area_struct *vma,
>  	 */
>  	*prev = NULL;	/* tell sys_madvise we drop mmap_sem */
>  	get_file(file);
> -	up_read(&current->mm->mmap_sem);
> +	up_read(&mm->mmap_sem);
>  	offset = (loff_t)(start - vma->vm_start)
>  			+ ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
>  	vfs_fadvise(file, offset, end - start, POSIX_FADV_WILLNEED);
>  	fput(file);
> -	down_read(&current->mm->mmap_sem);
> +	down_read(&mm->mmap_sem);
>  	return 0;
>  }
>  
> @@ -674,9 +675,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>  	}
>  out:
>  	if (nr_swap) {
> -		if (current->mm == mm)
> -			sync_mm_rss(mm);
> -
> +		sync_mm_rss(mm);
>  		add_mm_counter(mm, MM_SWAPENTS, nr_swap);
>  	}
>  	arch_leave_lazy_mmu_mode();

This seems like it is taking things in the opposite direction of the other
changes. sync_mm_rss will operate on current if I am not mistaken. I don't
think you would want to add the stats from current to the stats of the
task you are updating.

It might make sense to add a new function that would allow you to sync the
remote task stats by creaing a version of sync_mm_rss that also takes a
task pointer.

> @@ -756,6 +755,7 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
>  				  unsigned long start, unsigned long end,
>  				  int behavior)
>  {
> +	struct mm_struct *mm = vma->vm_mm;
>  	*prev = vma;
>  	if (!can_madv_lru_vma(vma))
>  		return -EINVAL;
> @@ -763,8 +763,8 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
>  	if (!userfaultfd_remove(vma, start, end)) {
>  		*prev = NULL; /* mmap_sem has been dropped, prev is stale */
>  
> -		down_read(&current->mm->mmap_sem);
> -		vma = find_vma(current->mm, start);
> +		down_read(&mm->mmap_sem);
> +		vma = find_vma(mm, start);
>  		if (!vma)
>  			return -ENOMEM;
>  		if (start < vma->vm_start) {

This piece of code has me wondering if it is valid to be using vma->mm at
the start of the function. I assume we are probably safe since we read the
mm value before the semaphore was released in userfaultfd_remove. It might
make more sense to just pass the task to the function and use task->mm-
>mmap_sem instead. 

It might be simpler, safer, and easier to review to just go through and
add the task struct as needed and then simply replace references to
current->mm with task->mm.

> @@ -818,6 +818,7 @@ static long madvise_remove(struct vm_area_struct *vma,
>  	loff_t offset;
>  	int error;
>  	struct file *f;
> +	struct mm_struct *mm = vma->vm_mm;
>  
>  	*prev = NULL;	/* tell sys_madvise we drop mmap_sem */
>  
> @@ -845,13 +846,13 @@ static long madvise_remove(struct vm_area_struct *vma,
>  	get_file(f);
>  	if (userfaultfd_remove(vma, start, end)) {
>  		/* mmap_sem was not released by userfaultfd_remove() */
> -		up_read(&current->mm->mmap_sem);
> +		up_read(&mm->mmap_sem);
>  	}
>  	error = vfs_fallocate(f,
>  				FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
>  				offset, end - start);
>  	fput(f);
> -	down_read(&current->mm->mmap_sem);
> +	down_read(&mm->mmap_sem);
>  	return error;
>  }
>  
> @@ -1044,7 +1045,8 @@ madvise_behavior_valid(int behavior)
>   *  -EBADF  - map exists, but area maps something that isn't a file.
>   *  -EAGAIN - a kernel resource was temporarily unavailable.
>   */
> -int do_madvise(unsigned long start, size_t len_in, int behavior)
> +int do_madvise(struct task_struct *task, unsigned long start,
> +					size_t len_in, int behavior)
>  {
>  	unsigned long end, tmp;
>  	struct vm_area_struct *vma, *prev;
> @@ -1053,6 +1055,7 @@ int do_madvise(unsigned long start, size_t len_in, int behavior)
>  	int write;
>  	size_t len;
>  	struct blk_plug plug;
> +	struct mm_struct *mm = task->mm;
>  
>  	start = untagged_addr(start);
>  
> @@ -1082,10 +1085,10 @@ int do_madvise(unsigned long start, size_t len_in, int behavior)
>  
>  	write = madvise_need_mmap_write(behavior);
>  	if (write) {
> -		if (down_write_killable(&current->mm->mmap_sem))
> +		if (down_write_killable(&mm->mmap_sem))
>  			return -EINTR;
>  	} else {
> -		down_read(&current->mm->mmap_sem);
> +		down_read(&mm->mmap_sem);
>  	}
>  
>  	/*
> @@ -1093,7 +1096,7 @@ int do_madvise(unsigned long start, size_t len_in, int behavior)
>  	 * ranges, just ignore them, but return -ENOMEM at the end.
>  	 * - different from the way of handling in mlock etc.
>  	 */
> -	vma = find_vma_prev(current->mm, start, &prev);
> +	vma = find_vma_prev(mm, start, &prev);
>  	if (vma && start > vma->vm_start)
>  		prev = vma;
>  
> @@ -1130,19 +1133,19 @@ int do_madvise(unsigned long start, size_t len_in, int behavior)
>  		if (prev)
>  			vma = prev->vm_next;
>  		else	/* madvise_remove dropped mmap_sem */
> -			vma = find_vma(current->mm, start);
> +			vma = find_vma(mm, start);
>  	}
>  out:
>  	blk_finish_plug(&plug);
>  	if (write)
> -		up_write(&current->mm->mmap_sem);
> +		up_write(&mm->mmap_sem);
>  	else
> -		up_read(&current->mm->mmap_sem);
> +		up_read(&mm->mmap_sem);
>  
>  	return error;
>  }
>  
>  SYSCALL_DEFINE3(madvise, unsigned long, start, size_t, len_in, int, behavior)
>  {
> -	return do_madvise(start, len_in, behavior);
> +	return do_madvise(current, start, len_in, behavior);
>  }