Re: [PATCH] mm: khugepaged: fix call hpage_collapse_scan_file() for anonymous vma

[Yang Shi <yang@os.amperecomputing.com>]

On 1/10/25 11:01 AM, Matthew Wilcox wrote:
> On Fri, Jan 10, 2025 at 10:04:42AM -0800, Yang Shi wrote:
>> On 1/9/25 8:31 PM, Matthew Wilcox wrote:
>>> On Thu, Jan 09, 2025 at 09:00:24AM -0800, Yang Shi wrote:
>>>> Thanks for catching this. It sounds a little bit weird to have vm_file for
>>>> an anonymous VMA. I'm not sure why we should keep such special case. It
>>>> seems shared mapping is treated as shmem file mapping. So can we set vm_file
>>>> to NULL when mmap'ing /dev/zero for private mapping? Something like:
>>>>
>>>> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
>>>> index 169eed162a7f..fc332efc5c11 100644
>>>> --- a/drivers/char/mem.c
>>>> +++ b/drivers/char/mem.c
>>>> @@ -527,6 +527,7 @@ static int mmap_zero(struct file *file, struct
>>>> vm_area_struct *vma)
>>>>           if (vma->vm_flags & VM_SHARED)
>>>>                   return shmem_zero_setup(vma);
>>>>           vma_set_anonymous(vma);
>>>> +       vma->vm_file = NULL;
>>>>           return 0;
>>>>    }
>>> I'm wary this might cause other bugs somewhere.  rc6 is a bit late to be
>>> introducing such a subtle change.
>> Thanks for the extra caution. Applying the proposed fix in khugepaged code
>> is fine to me either. We can try to kill the special case later.
>>
>> Looking at the code further, I think we should do more to make private
>> /dev/zero mapping an anonymous mapping:
> I'm still nervous about this.  We map device inodes in a lot of places.

Yes, we do. But I don't think this change actually changes the semantic 
of /dev/zero. Shared /dev/zero mapping is still treated as shmem 
mapping, private /dev/zero mapping is treated as anonymous mapping, but 
the current implementation is actually half-baked. It has NULL 
vma->vm_ops which is used to tell kernel whether it is an anonymous vma 
or not, but it also has valid vma->vm_file and vma->vm_pgoff as in file 
offset.

So this special case makes kernel has 3 types of VMA:
     - anonymous VMA: vm_ops is NULL, vm_file is NULL, vm_pgoff is the 
linear address pgoff
     - file VMA: vm_ops is *NOT* NULL, valid vm_file and vm_pgoff is 
index in file
     - private /dev/zero mapping VMA