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

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Date:   Fri, 5 Aug 2022 20:57:00 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.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 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.

-- 
Thanks,

David / dhildenb