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From:   Zi Yan <ziy@nvidia.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Yu Zhao <yuzhao@google.com>,
        "Yin, Fengwei" <fengwei.yin@intel.com>,
        Minchan Kim <minchan@kernel.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        willy@infradead.org, ryan.roberts@arm.com, shy828301@gmail.com,
        "Vishal Moola (Oracle)" <vishal.moola@gmail.com>
Subject: Re: [RFC PATCH] madvise: make madvise_cold_or_pageout_pte_range()
 support large folio
Date:   Fri, 14 Jul 2023 10:41:10 -0400
Message-ID: <6C3EFBC6-DC59-4167-B51A-7231A55FE1D5@nvidia.com>
In-Reply-To: <d024fa03-ca51-632b-9a01-bbdc75543f5d@redhat.com>
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On 14 Jul 2023, at 3:31, David Hildenbrand wrote:

> On 14.07.23 05:23, Yu Zhao wrote:
>> On Thu, Jul 13, 2023 at 9:10=E2=80=AFPM Yin, Fengwei <fengwei.yin@inte=
l.com> wrote:
>>>
>>>
>>>
>>> On 7/14/2023 10:08 AM, Yu Zhao wrote:
>>>> On Thu, Jul 13, 2023 at 9:06=E2=80=AFAM Yin Fengwei <fengwei.yin@int=
el.com> wrote:
>>>>>
>>>>> Current madvise_cold_or_pageout_pte_range() has two problems for
>>>>> large folio support:
>>>>>    - Using folio_mapcount() with large folio prevent large folio fr=
om
>>>>>      picking up.
>>>>>    - If large folio is in the range requested, shouldn't split it
>>>>>      in madvise_cold_or_pageout_pte_range().
>>>>>
>>>>> Fix them by:
>>>>>    - Use folio_estimated_sharers() with large folio
>>>>>    - If large folio is in the range requested, don't split it. Leav=
e
>>>>>      to page reclaim phase.
>>>>>
>>>>> For large folio cross boundaries of requested range, skip it if it'=
s
>>>>> page cache. Try to split it if it's anonymous folio. If splitting
>>>>> fails, skip it.
>>>>
>>>> For now, we may not want to change the existing semantic (heuristic)=
=2E
>>>> IOW, we may want to stick to the "only owner" condition:
>>>>
>>>>    - if (folio_mapcount(folio) !=3D 1)
>>>>    + if (folio_entire_mapcount(folio) ||
>>>>    +     (any_page_within_range_has_mapcount > 1))
>>>>
>>>> +Minchan Kim
>>> The folio_estimated_sharers() was discussed here:
>>> https://lore.kernel.org/linux-mm/20230118232219.27038-6-vishal.moola@=
gmail.com/
>>> https://lore.kernel.org/linux-mm/20230124012210.13963-2-vishal.moola@=
gmail.com/
>>>
>>> Yes. It's accurate to check each page of large folio. But it may be o=
ver killed in
>>> some cases (And I think madvise is one of the cases not necessary to =
be accurate.
>>> So folio_estimated_sharers() is enough. Correct me if I am wrong).
>>
>> I see. Then it's possible this is also what the original commit wants
>> to do -- Minchan, could you clarify?
>>
>> Regardless, I think we can have the following fix, potentially cc'ing =
stable:
>>
>> -  if (folio_mapcount(folio) !=3D 1)
>> +  if (folio_estimated_sharers(folio) !=3D 1)
>>
>> Sounds good?
>
> Adding to the discussion, currently the COW selftest always skips a PTE=
-mapped THP.
>
>
> For example:
>
> # [INFO] Anonymous memory tests in private mappings
> # [RUN] Basic COW after fork() ... with base page
> ok 1 No leak from parent into child
> # [RUN] Basic COW after fork() ... with swapped out base page
> ok 2 No leak from parent into child
> # [RUN] Basic COW after fork() ... with THP
> ok 3 No leak from parent into child
> # [RUN] Basic COW after fork() ... with swapped-out THP
> ok 4 No leak from parent into child
> # [RUN] Basic COW after fork() ... with PTE-mapped THP
> ok 5 No leak from parent into child
> # [RUN] Basic COW after fork() ... with swapped-out, PTE-mapped THP
> ok 6 # SKIP MADV_PAGEOUT did not work, is swap enabled?
> ...
>
>
> The commit that introduced that change is:
>
> commit 07e8c82b5eff8ef34b74210eacb8d9c4a2886b82
> Author: Vishal Moola (Oracle) <vishal.moola@gmail.com>
> Date:   Wed Dec 21 10:08:46 2022 -0800
>
>     madvise: convert madvise_cold_or_pageout_pte_range() to use folios
>
>     This change removes a number of calls to compound_head(), and saves=

>     1729 bytes of kernel text.
>
>
>
> folio_mapcount(folio) is wrong, because that never works on a PTE-mappe=
d THP (well, unless only a single subpage is still mapped ...).
>
> page_mapcount(folio) was wrong, because it ignored all other subpages, =
but at least it worked in some cases.
>
> folio_estimated_sharers(folio) is similarly wrong like page_mapcount(),=
 as it's essentially a page_mapcount() of the first subpage.
>
> (ignoring that a lockless mapcount-based check is always kind-of unreli=
able, but that's msotly acceptable for these kind of things)
>
>
> So, unfortunately, page_mapcount() / folio_estimated_sharers() is best =
we can do for now, but they miss to detect some cases of sharing of the f=
olio -- false negatives to detect sharing.
>
>
> Ideally we want something like folio_maybe_mapped_shared(), and get rid=
 of folio_estimated_sharers(), we better to guess the exact number, simpl=
y works towards an answer that tells us "yep, this may be mapped by multi=
ple sharers" vs. "no, this is definitely not mapped by multiple sharers".=

>
> The "mapped" part of it indicates that this does not catch all cases of=
 sharing. But it should handle most of the cases we care about.
>
>
> There, we can then implement something better than what folio_estimated=
_sharers() currently does:
>
> static inline bool folio_maybe_mapped_shared(folio)
> {
> 	if (likely(!folio_test_large(folio)))
> 		return atomic_read(&folio->_mapcount) > 0;
>
> 	/* Mapped multiple times via PMD? */
> 	if (folio_test_pmd_mappable(folio)
> 		return folio_entire_mapcount() > 1;
>
> 	/*
> 	 * First subpage is mapped multiple times (especially also via
> 	 * PMDs)?
>          */
> 	if (page_mapcount(folio_page(folio, 0) > 1)
> 		return true;
>
> 	/* TODO: also test last subpage? */
> 	=

> 	/* Definitely shared if we're mapping a page multiple times. */
> 	return folio_total_mapcount(folio) > folio_nr_pages(folio);
> }
>
> There are some more things we could optimize for.

Before jumping into the mapcount, I would like to get some clarification
on "sharer". Does it mean a page is mapped/shared by more than one page
table entry or is mapped/shared by more than one process? Your function
indicates it is the former, but for madvise_cold_or_pageout_pte_range(),
I am not sure that is what we want. What if user wants to page out a
page that is mapped by the same process twice? With current method
or any existing proposals, it will fail undesirably. It will only work
as expect with your creator proposal[1].

Other places like auto NUMA migration also use mapcount to check if
a page is mapped by multiple process with the assumption that a page will=

only be mapped by one process once.


[1] https://lore.kernel.org/linux-mm/6cec6f68-248e-63b4-5615-9e0f3f819a0a=
@redhat.com/

--
Best Regards,
Yan, Zi

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