Re: [PATCH 4/4] mm/madvise: remove redundant mmap_lock operations from process_madvise()

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* SeongJae Park <sj@kernel.org> [250206 01:15]:
> Optimize redundant mmap lock operations from process_madvise() by
> directly doing the mmap locking first, and then the remaining works for
> all ranges in the loop.
> 
> Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
> Signed-off-by: SeongJae Park <sj@kernel.org>

It might be worth calling out the drop/reacquire in the change log as
well as the comment like Lorenzo said.

Reviewed-by: Liam R. Howlett <howlett@gmail.com>

> ---
>  mm/madvise.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 31e5df75b926..5a0a1fc99d27 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1754,9 +1754,26 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
>  
>  	total_len = iov_iter_count(iter);
>  
> +	ret = madvise_lock(mm, behavior);
> +	if (ret)
> +		return ret;
> +
>  	while (iov_iter_count(iter)) {
> -		ret = do_madvise(mm, (unsigned long)iter_iov_addr(iter),
> -				 iter_iov_len(iter), behavior);
> +		unsigned long start = (unsigned long)iter_iov_addr(iter);
> +		size_t len_in = iter_iov_len(iter);
> +		size_t len;
> +
> +		if (!is_valid_madvise(start, len_in, behavior)) {
> +			ret = -EINVAL;
> +			break;
> +		}
> +
> +		len = PAGE_ALIGN(len_in);
> +		if (start + len == start)
> +			ret = 0;
> +		else
> +			ret = madvise_do_behavior(mm, start, len_in, len,
> +					behavior);
>  		/*
>  		 * An madvise operation is attempting to restart the syscall,
>  		 * but we cannot proceed as it would not be correct to repeat
> @@ -1772,12 +1789,17 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
>  				ret = -EINTR;
>  				break;
>  			}
> +
> +			/* Drop and reacquire lock to unwind race. */
> +			madvise_unlock(mm, behavior);
> +			madvise_lock(mm, behavior);
>  			continue;
>  		}
>  		if (ret < 0)
>  			break;
>  		iov_iter_advance(iter, iter_iov_len(iter));
>  	}
> +	madvise_unlock(mm, behavior);
>  
>  	ret = (total_len - iov_iter_count(iter)) ? : ret;
>  
> -- 
> 2.39.5