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

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Date:   Thu, 3 Aug 2023 16:10:17 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Ryan Roberts <ryan.roberts@....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>, Zi Yan <ziy@...dia.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4 3/3] mm: Batch-zap large anonymous folio PTE mappings

>>
>> 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.

>>
>>> +
>>> +    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.

-- 
Cheers,

David / dhildenb