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Subject: Re: [RFC PATCH v4 8/8] hugetlb: use new vma_lock for pmd sharing
 synchronization
To:     Mike Kravetz <mike.kravetz@oracle.com>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Muchun Song <songmuchun@bytedance.com>,
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References: <20220706202347.95150-1-mike.kravetz@oracle.com>
 <20220706202347.95150-9-mike.kravetz@oracle.com>
 <ddab06a9-ab81-5ebd-9273-c50744f6da60@huawei.com> <YuLLqbq1aOwFPsdi@monkey>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <2adbbbd2-51d2-744d-77b4-374fe651873b@huawei.com>
Date:   Fri, 29 Jul 2022 09:41:54 +0800
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On 2022/7/29 1:47, Mike Kravetz wrote:
> On 07/28/22 14:51, Miaohe Lin wrote:
snip
>>
>> Do we need to check &mm->mm_users == 0 here in case the address_space of corresponding process
>> has exited? In this case, mmdrop will drop the last reference and free the skipped_mm. So we will
>> use skipped_mm below after it's freed?
>>
> 
> Good point!
> I think we need to wait to drop since we want to hold the read lock.
> Will update.
> 
>>> +		vma = find_vma(skipped_mm, skipped_vm_start);
>>> +		if (!vma || vma->vm_file->f_mapping != mapping ||
>>
>> If skipped_vm_start is unmapped and remapped as a anon vma before we taking the mmap_read_lock,
>> vma->vm_file will be NULL?
>>
> 
> IIUC, vma->vm_file will always be set even for an anon vma.  The fault
> code depends on this.  See beginning of hugetlb_fault() where we
> unconditionally do:
> 
> mapping = vma->vm_file->f_mapping;

What if vma is non-hugetlb anon vma?

Thanks.