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

[Vlastimil Babka]

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;
>  
>