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Date:   Fri, 4 Mar 2022 18:25:49 +0100
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To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        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
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 <9dda55d5-eafa-3177-2a4c-32ccb7e146e3@google.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH mmotm] mm: delete __ClearPageWaiters()
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On 03.03.22 23:28, Hugh Dickins wrote:
> On Thu, 3 Mar 2022, David Hildenbrand wrote:
>> 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())
> 
> Hah, I'm showing my age there, or the patch's age: it's been rebranded
> frequently since then, with linux-next calling it free_zone_device_page(),
> as you kindly point out.  How long before it's free_zone_device_folio()?

:)

> 
>>> 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/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()?
> 
> I'm sure you could tell me a lot more about ZONE_DEVICE pages than I
> could ever tell you.  But, if they don't ever reach the main page freer,
> then they're in the same category as other pages not freed until reboot:
> any clearing of left-behind PG_waiters will be done by the next waker,
> not by reaching free_pages_prepare().  Does that really require special
> mention of ZONE_DEVICE pages here?  Would I do better just to remove
> the comment on PAGE_FLAGS_CHECK_AT_PREP being one of the clearers?

In this context we can consider ZONE_DEVICE pages just like any other
pages that, although getting freed, are not returned to the buddy, but
instead are returned to another pool. So PAGE_FLAGS_CHECK_AT_PREP won't
apply and free_pages_prepare() won't apply.

Another example would be hugetlb pages, that are returned to the hugetlb
pool, but not back to the buddy unless the huge page pool is shrunk.


So I feel like the underlying principle here is: we don't *care* if
PG_waiter is cleared when a page gets freed, because it will simply get
cleared by the next waker if it sticks around.

Then, I agree, we can just drop the comment regarding
PAGE_FLAGS_CHECK_AT_PREP and instead have something like


"
That's okay, it's a rare case and the next waker will just clear it.
Note that, depending on the page pool (buddy, ZONE_DEVICE, hugetlb), we
might clear the flag while freeing the page, however, this is not
required for correctness.
"


-- 
Thanks,

David / dhildenb