linux-kernel - Re: [PATCH v4 3/3] mm: Batch-zap large anonymous folio PTE mappings

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Date:   Thu, 03 Aug 2023 10:28:56 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4 3/3] mm: Batch-zap large anonymous folio PTE mappings

On 3 Aug 2023, at 10:15, Ryan Roberts wrote:

> On 03/08/2023 15:10, David Hildenbrand wrote:
>>>>
>>>> With this patch, you'll might suddenly have mapcount > refcount for a folio, or
>>>> am I wrong?
>>>
>>> Yes you would. Does that break things?
>>>
>>
>> It is problematic whenever you want to check for additional page references that
>> are not from mappings (i.e., GUP refs/pins or anything else)
>>
>> One example lives in KSM code (!compound only):
>>
>> page_mapcount(page) + 1 + swapped != page_count(page)
>>
>> Another one in compaction code:
>>
>> if (!mapping && (folio_ref_count(folio) - 1) > folio_mapcount(folio))
>>
>> And another one in khugepaged (is_refcount_suitable)
>>
>> ... and in THP split can_split_folio() (although that can deal with false
>> positives and false negatives).
>>
>>
>> We want to avoid detecting "no other references" if there *are* other
>> references. Detecting "there are other references" although there are not is
>> usually better.
>>
>>
>> Assume you have mapcount > refcount for some time due to concurrent unmapping,
>> AND some unrelated reference. You would suddenly pass these checks (mapcount ==
>> refcount) and might not detect other references.
>
> OK. I'll rework with the 2 loop approach, assuming I can calculate the number of
> free slots in the mmu_gather ahead of time.
>
>
>>
>>>>
>>>>> +
>>>>> +    for (i = 0; i < nr_pages;) {
>>>>> +        ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
>>>>> +        tlb_remove_tlb_entry(tlb, pte, addr);
>>>>> +        zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
>>>>> +        full = __tlb_remove_page(tlb, page, 0);
>>>>> +
>>>>> +        if (unlikely(page_mapcount(page) < 1))
>>>>> +            print_bad_pte(vma, addr, ptent, page);
>>>>
>>>> Can we avoid new users of page_mapcount() outside rmap code, please? :)
>>>
>>> Sure. This is just trying to replicate the same diagnstics that's done on the
>>> non-batched path. I'm happy to remove it.
>>
>> Spotted it afterwards in the existing code already, so you're effetively not
>> adding new ones.

I agree that we should keep the original logic flow and use the 2 loop approach.
Otherwise, the (unlikely(page_mapcount(page) < 1)) check might not work as
expected, since the page mapcount is decreased after this check in your code.


--
Best Regards,
Yan, Zi

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