linux-kernel - Re: [RFC PATCH RESEND 07/28] kernel/fork: mark VMAs as locked before copying pages during fork

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Date:   Fri, 9 Sep 2022 15:27:30 +0200
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, mhocko@...e.com, vbabka@...e.cz,
        hannes@...xchg.org, mgorman@...e.de, dave@...olabs.net,
        willy@...radead.org, liam.howlett@...cle.com, peterz@...radead.org,
        laurent.dufour@...ibm.com, paulmck@...nel.org, luto@...nel.org,
        songliubraving@...com, peterx@...hat.com, david@...hat.com,
        dhowells@...hat.com, hughd@...gle.com, bigeasy@...utronix.de,
        kent.overstreet@...ux.dev, rientjes@...gle.com,
        axelrasmussen@...gle.com, joelaf@...gle.com, minchan@...gle.com,
        kernel-team@...roid.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH RESEND 07/28] kernel/fork: mark VMAs as locked before
 copying pages during fork

Le 09/09/2022 à 01:57, Suren Baghdasaryan a écrit :
> On Tue, Sep 6, 2022 at 7:38 AM Laurent Dufour <ldufour@...ux.ibm.com> wrote:
>>
>> Le 01/09/2022 à 19:34, Suren Baghdasaryan a écrit :
>>> Protect VMAs from concurrent page fault handler while performing
>>> copy_page_range for VMAs having VM_WIPEONFORK flag set.
>>
>> I'm wondering why is that necessary.
>> The copied mm is write locked, and the destination one is not reachable.
>> If any other readers are using the VMA, this is only for page fault handling.
> 
> Correct, this is done to prevent page faulting in the VMA being
> duplicated. I assume we want to prevent the pages in that VMA from
> changing when we are calling copy_page_range(). Am I wrong?

If a page is faulted while copy_page_range() is in progress, the page may
not be backed on the child side (PTE lock should protect the copy, isn't it).
Is that a real problem? It will be backed later if accessed on the child side.
Maybe the per process pages accounting could be incorrect...

> 
>> I should have miss something because I can't see any need to mark the lock
>> VMA here.
>>
>>> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
>>> ---
>>>  kernel/fork.c | 4 +++-
>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>> index bfab31ecd11e..1872ad549fed 100644
>>> --- a/kernel/fork.c
>>> +++ b/kernel/fork.c
>>> @@ -709,8 +709,10 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>>>               rb_parent = &tmp->vm_rb;
>>>
>>>               mm->map_count++;
>>> -             if (!(tmp->vm_flags & VM_WIPEONFORK))
>>> +             if (!(tmp->vm_flags & VM_WIPEONFORK)) {
>>> +                     vma_mark_locked(mpnt);
>>>                       retval = copy_page_range(tmp, mpnt);
>>> +             }
>>>
>>>               if (tmp->vm_ops && tmp->vm_ops->open)
>>>                       tmp->vm_ops->open(tmp);
>>