linux-kernel - Re: [PATCH] mm: mlock: remove lru_add_drain

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Date:   Thu, 19 Oct 2017 14:32:06 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Minchan Kim <minchan@...nel.org>,
        Yisheng Xie <xieyisheng1@...wei.com>,
        Ingo Molnar <mingo@...nel.org>,
        Greg Thelen <gthelen@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: mlock: remove lru_add_drain_all()

On Wed 18-10-17 16:17:30, Shakeel Butt wrote:
> Recently we have observed high latency in mlock() in our generic
> library and noticed that users have started using tmpfs files even
> without swap and the latency was due to expensive remote LRU cache
> draining.

some numbers would be really nice
 
> Is lru_add_drain_all() required by mlock()? The answer is no and the
> reason it is still in mlock() is to rapidly move mlocked pages to
> unevictable LRU.

Is this really true? lru_add_drain_all will flush the previously cached
LRU pages. We are not flushing after the pages have been faulted in so
this might not do anything wrt. mlocked pages, right?

> Without lru_add_drain_all() the mlocked pages which
> were on pagevec at mlock() time will be moved to evictable LRUs but
> will eventually be moved back to unevictable LRU by reclaim. So, we
> can safely remove lru_add_drain_all() from mlock(). Also there is no
> need for local lru_add_drain() as it will be called deep inside
> __mm_populate() (in follow_page_pte()).

Anyway, I do agree that lru_add_drain_all here is pointless. Either we
should drain after the memory has been faulted in and mlocked or not at
all. So the patch looks good to me I am just not sure about the
changelog.
 
> Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
> ---
>  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;
>  
> -- 
> 2.15.0.rc1.287.g2b38de12cc-goog
> 

-- 
Michal Hocko
SUSE Labs