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

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Date:   Fri, 5 Aug 2022 13:48:35 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Peter Xu <peterx@...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 20:57, David Hildenbrand wrote:
> On 05.08.22 20:33, Mike Kravetz wrote:
> > On 08/05/22 20:25, David Hildenbrand wrote:
> >> On 05.08.22 20:23, Mike Kravetz wrote:
> >>> 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.
> >>
> >>
> >> ... does your config have CONFIG_MEM_SOFT_DIRTY enabled?
> >>
> > 
> > No, Duh!
> > 
> > FYI - Some time back, I started looking at adding soft dirty support for
> > hugetlb mappings.  I did not finish that work.  But, I seem to recall
> > places where code was operating on hugetlb mappings when perhaps it should
> > not.
> > 
> > Perhaps, it would also be good to just disable soft dirty for hugetlb at
> > the source?
> 
> I thought about that as well. But I came to the conclusion that without
> patch #2, hugetlb VMAs cannot possibly support write-notify, so there is
> no need to bother in vma_wants_writenotify() at all.
> 
> The "root" would be places where we clear VM_SOFTDIRTY. That should only
> be fs/proc/task_mmu.c:clear_refs_write() IIRC.
> 
> So I don't particularly care, I consider this patch a bit cleaner and
> more generic, but I can adjust clear_refs_write() instead of there is a
> preference.
> 

After a closer look, I agree that this may be the simplest/cleanest way to
proceed.  I was going to suggest that you note hugetlb does not support
softdirty, but see you did in the comment.

Acked-by: Mike Kravetz <mike.kravetz@...cle.com>

-- 
Mike Kravetz