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

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Date:   Fri, 5 Aug 2022 19:13:47 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.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 Fri, Aug 05, 2022 at 01:48:35PM -0700, Mike Kravetz wrote:
> 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>

Filtering out hugetlbfs in vma_wants_writenotify() is still a bit hard to
follow to me, since it's not clear why hugetlbfs never wants writenotify.

If it's only about soft-dirty, we could have added the hugetlbfs check into
vma_soft_dirty_enabled(), then I think it'll achieve the same thing and
much clearer - with the soft-dirty check constantly returning false for it,
hugetlbfs shared vmas should have vma_wants_writenotify() naturally return
0 already.

For the long term - shouldn't we just enable soft-dirty for hugetlbfs?  I
remember Mike used to have that in todo.  Since we've got patch 2 already,
I feel like that's really much close (is the only missing piece the clear
refs write part? or maybe some more that I didn't notice).

Then patch 1 (or IMHO equivalant check in vma_soft_dirty_enabled(), but
maybe in stable trees we don't have vma_soft_dirty_enabled then it's
exactly patch 1) can be a stable-only backport just to avoid the bug from
triggering.

Thanks,

-- 
Peter Xu