linux-kernel - Re: [PATCH 6/8] hugetlb: add vma based lock for pmd sharing

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Date:   Thu, 8 Sep 2022 10:04:21 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
CC:     Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...e.com>, Peter Xu <peterx@...hat.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Prakash Sangappa <prakash.sangappa@...cle.com>,
        James Houghton <jthoughton@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Ray Fucillo <Ray.Fucillo@...ersystems.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/8] hugetlb: add vma based lock for pmd sharing

On 2022/9/8 4:50, Mike Kravetz wrote:
> On 08/29/22 15:24, Mike Kravetz wrote:
>> On 08/27/22 17:30, Miaohe Lin wrote:
>>> On 2022/8/25 1:57, Mike Kravetz wrote:
>>>> Allocate a rw semaphore and hang off vm_private_data for
>>>> synchronization use by vmas that could be involved in pmd sharing.  Only
>>>> add infrastructure for the new lock here.  Actual use will be added in
>>>> subsequent patch.
>>>>
>>>> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
>>>
>>> <snip>
>>>
>>>> +static void hugetlb_vma_lock_free(struct vm_area_struct *vma)
>>>> +{
>>>> +	/*
>>>> +	 * Only present in sharable vmas.  See comment in
>>>> +	 * __unmap_hugepage_range_final about the neeed to check both
>>>
>>> s/neeed/need/
>>>
>>>> +	 * VM_SHARED and VM_MAYSHARE in free path
>>>
>>> I think there might be some wrong checks around this patch. As above comment said, we
>>> need to check both flags, so we should do something like below instead?
>>>
>>> 	if (!(vma->vm_flags & (VM_MAYSHARE | VM_SHARED) == (VM_MAYSHARE | VM_SHARED)))
>>>
>>>> +	 */
>>
>> Thanks.  I will update.
>>
>>>> +	if (!vma || !(vma->vm_flags & (VM_MAYSHARE | VM_SHARED)))
>>>> +		return;
> 
> I think you misunderstood the comment which I admit was not very clear.  And,
> I misunderstood your suggestion.  I believe the code is correct as it.  Here
> is the proposed update comment/code:
> 
> 	/*
> 	 * Only present in sharable vmas.  See comment in
> 	 * __unmap_hugepage_range_final about how VM_SHARED could
> 	 * be set without VM_MAYSHARE.  As a result, we need to
> 	 * check if either is set in the free path.
> 	 */
> 	if (!vma || !(vma->vm_flags & (VM_MAYSHARE | VM_SHARED)))
> 		return;
> 
> Hopefully, that makes more sense.

Somewhat confusing. Thanks for clarifying, Mike.

Thanks,
Miaohe Lin