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

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Date:   Wed, 7 Sep 2022 13:50:46 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Miaohe Lin <linmiaohe@...wei.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 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.
-- 
Mike Kravetz