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Date:   Thu, 20 Aug 2020 08:18:27 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Yu Zhao <yuzhao@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Huang Ying <ying.huang@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Yang Shi <yang.shi@linux.alibaba.com>, Qian Cai <cai@lca.pw>,
        Mel Gorman <mgorman@suse.de>,
        Nicholas Piggin <npiggin@gmail.com>,
        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
        Hugh Dickins <hughd@google.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: Re: [PATCH v2 3/3] mm: remove superfluous __ClearPageWaiters()
Message-ID: <20200820061652.GX5422@dhcp22.suse.cz>
References: <20200818184704.3625199-1-yuzhao@google.com>
 <20200818184704.3625199-3-yuzhao@google.com>
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On Tue 18-08-20 12:47:04, Yu Zhao wrote:
> Presumably __ClearPageWaiters() was added to follow the previously
> removed __ClearPageActive() pattern.

I do not think so. Please have a look at 62906027091f ("mm: add
PageWaiters indicating tasks are waiting for a page bit") and a
discussion when the patch has been proposed. Sorry I do not have a link
handy but I do remember that the handling was quite subtle.
 
> Only flags that are in PAGE_FLAGS_CHECK_AT_FREE needs to be properly
> cleared because otherwise we think there may be some kind of leak.
> PG_waiters is not one of those flags and leaving the clearing to
> PAGE_FLAGS_CHECK_AT_PREP is more appropriate.

What is the point of this patch in the first place? Page waiters is
quite subtle and I wouldn't touch it without having a very good reason.

> Signed-off-by: Yu Zhao <yuzhao@google.com>
> ---
>  include/linux/page-flags.h | 2 +-
>  mm/filemap.c               | 2 ++
>  mm/memremap.c              | 2 --
>  mm/swap.c                  | 3 ---
>  4 files changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 6be1aa559b1e..dba80a2bdfba 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -318,7 +318,7 @@ static inline int TestClearPage##uname(struct page *page) { return 0; }
>  	TESTSETFLAG_FALSE(uname) TESTCLEARFLAG_FALSE(uname)
>  
>  __PAGEFLAG(Locked, locked, PF_NO_TAIL)
> -PAGEFLAG(Waiters, waiters, PF_ONLY_HEAD) __CLEARPAGEFLAG(Waiters, waiters, PF_ONLY_HEAD)
> +PAGEFLAG(Waiters, waiters, PF_ONLY_HEAD)
>  PAGEFLAG(Error, error, PF_NO_TAIL) TESTCLEARFLAG(Error, error, PF_NO_TAIL)
>  PAGEFLAG(Referenced, referenced, PF_HEAD)
>  	TESTCLEARFLAG(Referenced, referenced, PF_HEAD)
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 1aaea26556cc..75240c7ef73f 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1079,6 +1079,8 @@ static void wake_up_page_bit(struct page *page, int bit_nr)
>  		 * other pages on it.
>  		 *
>  		 * That's okay, it's a rare case. The next waker will clear it.
> +		 * Otherwise the bit will be cleared by PAGE_FLAGS_CHECK_AT_PREP
> +		 * when the page is being freed.
>  		 */
>  	}
>  	spin_unlock_irqrestore(&q->lock, flags);
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 3a06eb91cb59..a9d02ffaf9e3 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -451,8 +451,6 @@ void free_devmap_managed_page(struct page *page)
>  		return;
>  	}
>  
> -	__ClearPageWaiters(page);
> -
>  	mem_cgroup_uncharge(page);
>  
>  	/*
> diff --git a/mm/swap.c b/mm/swap.c
> index 999a84dbe12c..40bf20a75278 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -90,7 +90,6 @@ static void __page_cache_release(struct page *page)
>  		del_page_from_lru_list(page, lruvec, page_off_lru(page));
>  		spin_unlock_irqrestore(&pgdat->lru_lock, flags);
>  	}
> -	__ClearPageWaiters(page);
>  }
>  
>  static void __put_single_page(struct page *page)
> @@ -900,8 +899,6 @@ void release_pages(struct page **pages, int nr)
>  			del_page_from_lru_list(page, lruvec, page_off_lru(page));
>  		}
>  
> -		__ClearPageWaiters(page);
> -
>  		list_add(&page->lru, &pages_to_free);
>  	}
>  	if (locked_pgdat)
> -- 
> 2.28.0.220.ged08abb693-goog
> 

-- 
Michal Hocko
SUSE Labs