linux-kernel - Re: [PATCH RFC] mm: entirely reuse the whole anon mTHP in do_wp

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Message-ID: <5d0b6380-833d-449e-a6b2-e9b023832832@redhat.com>
Date: Sat, 31 Aug 2024 12:14:38 +0200
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>,
 Chuanhua Han <hanchuanhua@...o.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Ryan Roberts <ryan.roberts@....com>, Zi Yan <ziy@...dia.com>,
 Chris Li <chrisl@...nel.org>, Kairui Song <kasong@...cent.com>,
 Kalesh Singh <kaleshsingh@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH RFC] mm: entirely reuse the whole anon mTHP in do_wp_page

On 31.08.24 12:09, Barry Song wrote:
> On Sat, Aug 31, 2024 at 9:59 PM David Hildenbrand <david@...hat.com> wrote:
>>
>>
>>> +             idx = folio_page_idx(folio, vmf->page);
>>> +             folio_start = address - idx * PAGE_SIZE;
>>> +             folio_end = folio_start + nr * PAGE_SIZE;
>>> +
>>> +             if (unlikely(folio_start < max(address & PMD_MASK, vma->vm_start)))
>>> +                     return false;
>>> +             if (unlikely(folio_end > pmd_addr_end(address, vma->vm_end)))
>>> +                     return false;
>>> +             folio_ptep = vmf->pte - idx;
>>> +             folio_pte = ptep_get(folio_ptep);
>>> +             if (!pte_present(folio_pte) || pte_pfn(folio_pte) != folio_pfn(folio))
>>> +                     return false;
>>> +             if (folio_pte_batch(folio, folio_start, folio_ptep, folio_pte, nr, 0,
>>> +                             NULL, NULL, NULL) != nr)
>>> +                     return false;
>>> +             if (folio_mapcount(folio) != nr)
>>> +                     return false;
>>
>> BTW, you're not checking against the refcount (and it's all a bit racy
>> on concurrent unmapping!). So you're re-introducing the vmsplice
>> child->parent attak.
> 
> i don't quite understand this, you mean the below is not enough?
> 

Ah! You use the fallthrough, sorry I missed that!

You're not handling the swapcache references "correctly" (would have one 
reference per page), but the final check would be correct.

Yes, that should not allow for false positives here.

-- 
Cheers,

David / dhildenb