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Message-ID: <a2f6aa01-8003-6791-718d-3898f77ef643@redhat.com>
Date:   Thu, 3 Mar 2022 09:54:34 +0100
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Subject: Re: [PATCH mmotm] mm: delete __ClearPageWaiters()
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To:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Yu Zhao <yuzhao@google.com>, Yang Shi <shy828301@gmail.com>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <351df0af-78f2-c20-2a6d-e5f978e5ca1@google.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <351df0af-78f2-c20-2a6d-e5f978e5ca1@google.com>
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On 03.03.22 02:56, Hugh Dickins wrote:
> The PG_waiters bit is not included in PAGE_FLAGS_CHECK_AT_FREE, and
> vmscan.c's free_unref_page_list() callers rely on that not to generate
> bad_page() alerts.  So __page_cache_release() and release_pages() (and
> the presumably copy-and-pasted put_zone_device_private_or_public_page())
> are redundant and misleading to make a special point of clearing it (as
> the "__" implies, it could only safely be used on the freeing path).
> 
> Delete __ClearPageWaiters().  Remark on this in one of the "possible"
> comments in wake_up_page_bit(), and delete the superfluous comments.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
> We've used this since 2018, and I see Yu Zhao posted similar in 2020:
> https://lore.kernel.org/linux-mm/20200818184704.3625199-3-yuzhao@google.com/
> I couldn't join in at that time, but think its reception was over-cautious.
> 
>  include/linux/page-flags.h |  2 +-
>  mm/filemap.c               | 22 +++++++---------------
>  mm/memremap.c              |  2 --
>  mm/swap.c                  |  4 ----
>  4 files changed, 8 insertions(+), 22 deletions(-)
> 
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -481,7 +481,7 @@ static unsigned long *folio_flags(struct folio *folio, unsigned n)
>  	TESTSETFLAG_FALSE(uname, lname) TESTCLEARFLAG_FALSE(uname, lname)
>  
>  __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)
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1179,24 +1179,16 @@ static void folio_wake_bit(struct folio *folio, int bit_nr)
>  	}
>  
>  	/*
> -	 * It is possible for other pages to have collided on the waitqueue
> -	 * hash, so in that case check for a page match. That prevents a long-
> -	 * term waiter
> +	 * It's possible to miss clearing waiters here, when we woke our page
> +	 * waiters, but the hashed waitqueue has waiters for other pages on it.
>  	 *
> -	 * It is still possible to miss a case here, when we woke page waiters
> -	 * and removed them from the waitqueue, but there are still other
> -	 * page waiters.
> +	 * That's okay, it's a rare case. The next waker will clear it. Or,
> +	 * it might be left set until the page is freed: when it's masked off
> +	 * with others in PAGE_FLAGS_CHECK_AT_PREP, by free_pages_prepare().
>  	 */

Does that also apply to ZONE_DEVICE pages via free_zone_device_page()?


-- 
Thanks,

David / dhildenb