linux-kernel - Re: [PATCH v2 1/5] mm/shmem: Unconditionally set pte dirty in mfill_atomic_install

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Date:   Fri, 3 Sep 2021 22:02:59 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jerome Glisse <jglisse@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Alistair Popple <apopple@...dia.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>
Subject: Re: [PATCH v2 1/5] mm/shmem: Unconditionally set pte dirty in
 mfill_atomic_install_pte

On 03.09.21 22:00, Peter Xu wrote:
> On Fri, Sep 03, 2021 at 09:42:34AM +0200, David Hildenbrand wrote:
>> On 02.09.21 22:17, Peter Xu wrote:
>>> It was conditionally done previously, as there's one shmem special case that we
>>> use SetPageDirty() instead.  However that's not necessary and it should be
>>> easier and cleaner to do it unconditionally in mfill_atomic_install_pte().
>>>
>>> The most recent discussion about this is here, where Hugh explained the history
>>> of SetPageDirty() and why it's possible that it's not required at all:
>>>
>>> https://lore.kernel.org/lkml/alpine.LSU.2.11.2104121657050.1097@eggly.anvils/
>>>
>>> Currently mfill_atomic_install_pte() has three callers:
>>>
>>>           1. shmem_mfill_atomic_pte
>>>           2. mcopy_atomic_pte
>>>           3. mcontinue_atomic_pte
>>>
>>> After the change: case (1) should have its SetPageDirty replaced by the dirty
>>> bit on pte (so we unify them together, finally), case (2) should have no
>>> functional change at all as it has page_in_cache==false, case (3) may add a
>>> dirty bit to the pte.  However since case (3) is UFFDIO_CONTINUE for shmem,
>>> it's merely 100% sure the page is dirty after all, so should not make a real
>>> difference either.
>>
>> Would it be worth adding VM_BUG_ON() to make sure that "100%" is really the
>> case?
> 
> I won't be able to make it 100% sure (and that's where I put it "merely").  The
> example discussed between Axel and me in the other thread could be an outlier
> (when two processes, uffd target, and uffd minor resolver, map the region as
> RO), it's just that neither do I think that's a great matter, nor do I think it
> would be worth a BUG_ON(), not to mention we use BUG_ON so carefully.

Agreed then, if we really expect there are corner cases and that the 
corner cases are fine!

(VM_BUG_ON() could have helped to catch these while testing)

-- 
Thanks,

David / dhildenb