Re: [PATCH v2] mm: mlock: remove lru_add_drain_all()

From: Vlastimil Babka
Date: Mon Oct 30 2017 - 05:12:54 EST


On 10/20/2017 12:25 AM, Shakeel Butt wrote:
> lru_add_drain_all() is not required by mlock() and it will drain
> everything that has been cached at the time mlock is called. And
> that is not really related to the memory which will be faulted in
> (and cached) and mlocked by the syscall itself.
>
> Without lru_add_drain_all() the mlocked pages can remain on pagevecs
> and be moved to evictable LRUs. However they will eventually be moved
> back to unevictable LRU by reclaim. So, we can safely remove
> lru_add_drain_all() from mlock syscall. Also there is no need for
> local lru_add_drain() as it will be called deep inside __mm_populate()
> (in follow_page_pte()).
>
> On larger machines the overhead of lru_add_drain_all() in mlock() can
> be significant when mlocking data already in memory. We have observed
> high latency in mlock() due to lru_add_drain_all() when the users
> were mlocking in memory tmpfs files.
>
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
> Changelog since v1:
> - updated commit message
>
> mm/mlock.c | 5 -----
> 1 file changed, 5 deletions(-)
>
> diff --git a/mm/mlock.c b/mm/mlock.c
> index dfc6f1912176..3ceb2935d1e0 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -669,8 +669,6 @@ static __must_check int do_mlock(unsigned long start, size_t len, vm_flags_t fla
> if (!can_do_mlock())
> return -EPERM;
>
> - lru_add_drain_all(); /* flush pagevec */
> -
> len = PAGE_ALIGN(len + (offset_in_page(start)));
> start &= PAGE_MASK;
>
> @@ -797,9 +795,6 @@ SYSCALL_DEFINE1(mlockall, int, flags)
> if (!can_do_mlock())
> return -EPERM;
>
> - if (flags & MCL_CURRENT)
> - lru_add_drain_all(); /* flush pagevec */
> -
> lock_limit = rlimit(RLIMIT_MEMLOCK);
> lock_limit >>= PAGE_SHIFT;
>
>