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From:   Henry Burns <henryburns@google.com>
Date:   Mon, 15 Jul 2019 12:56:38 -0700
Message-ID: <CAGQXPTh-Z664T3Uxak-CiRn6Mc-s=esRzURLpwQaN+v0RgxFyg@mail.gmail.com>
Subject: Re: [PATCH] mm/z3fold.c: Reinitialize zhdr structs after migration
To:     Vitaly Wool <vitalywool@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Vitaly Vul <vitaly.vul@sony.com>,
        Shakeel Butt <shakeelb@google.com>,
        Jonathan Adams <jwadams@google.com>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org
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>>
>> z3fold_page_migration() calls memcpy(new_zhdr, zhdr, PAGE_SIZE).
>> However, zhdr contains fields that can't be directly coppied over (ex:
>> list_head, a circular linked list). We only need to initialize the
>> linked lists in new_zhdr, as z3fold_isolate_page() already ensures
>> that these lists are empty.
>>
>> Additionally it is possible that zhdr->work has been placed in a
>> workqueue. In this case we shouldn't migrate the page, as zhdr->work
>> references zhdr as opposed to new_zhdr.
>>
>> Fixes: bba4c5f96ce4 ("mm/z3fold.c: support page migration")
>> Signed-off-by: Henry Burns <henryburns@google.com>
>> ---
>>  mm/z3fold.c | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> diff --git a/mm/z3fold.c b/mm/z3fold.c
>> index 42ef9955117c..9da471bcab93 100644
>> --- a/mm/z3fold.c
>> +++ b/mm/z3fold.c
>> @@ -1352,12 +1352,22 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
>>                 z3fold_page_unlock(zhdr);
>>                 return -EBUSY;
>>         }
>> +       if (work_pending(&zhdr->work)) {
>> +               z3fold_page_unlock(zhdr);
>> +               return -EAGAIN;
>> +       }
>>         new_zhdr = page_address(newpage);
>>         memcpy(new_zhdr, zhdr, PAGE_SIZE);
>>         newpage->private = page->private;
>>         page->private = 0;
>>         z3fold_page_unlock(zhdr);
>>         spin_lock_init(&new_zhdr->page_lock);
>> +       INIT_WORK(&new_zhdr->work, compact_page_work);
>> +       /*
>> +        * z3fold_page_isolate() ensures that this list is empty, so we only
>> +        * have to reinitialize it.
>> +        */
>
>
> On the nitpicking side, we seem to have ensured that directly in migrate :) Looks OK to me otherwise.
Ok, I see it happens in the call to do_compact_page(). Got it, new
patch coming out now.

>
> ~Vitaly
>
>> +       INIT_LIST_HEAD(&new_zhdr->buddy);
>>         new_mapping = page_mapping(page);
>>         __ClearPageMovable(page);
>>         ClearPagePrivate(page);
>> --
>> 2.22.0.510.g264f2c817a-goog
>>