linux-kernel - Re: [PATCH v1 1/2] mm/hugetlb: fix hugetlb not supporting write-notify

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Date:   Fri, 5 Aug 2022 11:23:26 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <songmuchun@...edance.com>,
        Peter Feiner <pfeiner@...gle.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v1 1/2] mm/hugetlb: fix hugetlb not supporting
 write-notify

On 08/05/22 14:14, Peter Xu wrote:
> On Fri, Aug 05, 2022 at 01:03:28PM +0200, David Hildenbrand wrote:
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 61e6135c54ef..462a6b0344ac 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1683,6 +1683,13 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
> >  	if ((vm_flags & (VM_WRITE|VM_SHARED)) != ((VM_WRITE|VM_SHARED)))
> >  		return 0;
> >  
> > +	/*
> > +	 * Hugetlb does not require/support writenotify; especially, it does not
> > +	 * support softdirty tracking.
> > +	 */
> > +	if (is_vm_hugetlb_page(vma))
> > +		return 0;
> 
> I'm kind of confused here..  you seems to be fixing up soft-dirty for
> hugetlb but here it's explicitly forbidden.
> 
> Could you explain a bit more on why this patch is needed if (assume
> there'll be a working) patch 2 being provided?
> 

No comments on the patch, but ...

Since it required little thought, I ran the test program on next-20220802 and
was surprised that the issue did not recreate.  Even added a simple printk
to make sure we were getting into vma_wants_writenotify with a hugetlb vma.
We were.

I can easily recreate with an older Fedora kernel.

Will be trying to understand why this is the case.
-- 
Mike Kravetz