linux-kernel - Re: [PATCH 4/4] mm/madvise: remove redundant mmap_lock operations from process

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Message-ID: <048896fe-69cf-479c-a89c-0242da88f84e@lucifer.local>
Date: Thu, 6 Feb 2025 13:04:53 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: SeongJae Park <sj@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Shakeel Butt <shakeel.butt@...ux.dev>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 4/4] mm/madvise: remove redundant mmap_lock operations
 from process_madvise()

On Wed, Feb 05, 2025 at 10:15:17PM -0800, SeongJae Park wrote:
> 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@...ux.dev>
> Signed-off-by: SeongJae Park <sj@...nel.org>

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

But please fixup the comment as below. Maybe Andrew could do it if I am
giving enough info to go on, otherwise it could be a quick fix-patch? I
attach a lazy attempt at a fix-patch in case that's useful.

> ---
>  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

This comment is now incorrect, please fix up the 2nd paragraph so it is
something like:

		/*
		 * An madvise operation is attempting to restart the syscall,
		 * but we cannot proceed as it would not be correct to repeat
		 * the operation in aggregate, and would be surprising to the
		 * user.
		 *
		 * We drop and reacquire locks so it is safe to just loop and
		 * try again. We check for fatal signals in case we need exit
		 * early anyway.
		 */

See below fixpatch if that's easier...

> @@ -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);

Thanks for adding!

>  			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

For convenience I attach a lazy, untested fixpatch which may or may not work :)

----8<----
>From 86e99a658a5e0195050d6bb9e19975f54bf14e7a Mon Sep 17 00:00:00 2001
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Date: Thu, 6 Feb 2025 13:02:52 +0000
Subject: [PATCH] foo

---
 mm/madvise.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index c8e28d51978a..ab5f5da1571f 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1725,7 +1725,7 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
 		 * the operation in aggregate, and would be surprising to the
 		 * user.
 		 *
-		 * As we have already dropped locks, it is safe to just loop and
+		 * We drop and reacquire locks so it is safe to just loop and
 		 * try again. We check for fatal signals in case we need exit
 		 * early anyway.
 		 */
--
2.48.1