Re: [PATCH] mm: Allow shmem mappings with MREMAP_DONTUNMAP

[Dmitry Safonov]

Hi Brian, Hugh,

On 3/16/21 7:18 PM, Brian Geffon wrote:
> Hi Hugh,
> Thanks for this suggestion, responses in line.
> 
>> A better patch would say:
>>
>> -       if (flags & MREMAP_DONTUNMAP && (!vma_is_anonymous(vma) ||
>> -                       vma->vm_flags & VM_SHARED))
>> +       if ((flags & MREMAP_DONTUNMAP) &&
>> +           (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP)))
>>                 return ERR_PTR(-EINVAL);
>>
>> VM_DONTEXPAND is what has long been used on special mappings, to prevent
>> surprises from mremap changing the size of the mapping: MREMAP_DONTUNMAP
>> introduced a different way of expanding the mapping, so VM_DONTEXPAND
>> still seems a reasonable name (I've thrown in VM_PFNMAP there because
>> it's in the VM_DONTEXPAND test lower down: for safety I guess, and best
>> if both behave the same - though one says -EINVAL and the other -EFAULT).
> 
> I like this idea and am happy to mail a new patch. I think it may make
> sense to bring the lower block up here so that it becomes more clear
> that it's not duplicate code and that the MREMAP_DONTUNMAP case
> returns -EINVAL and other cases return -EFAULT. I wonder if the
> -EFAULT error code would have made more sense from the start for both
> cases, do you have any thoughts on changing the error code at this
> point?
> 
>> With that VM_DONTEXPAND check in, Dmitry's commit cd544fd1dc92
>> ("mremap: don't allow MREMAP_DONTUNMAP on special_mappings and aio")
>> can still be reverted (as you agreed on 28th December), even though
>> vma_is_anonymous() will no longer protect it.
> 
> I agree and if Dmitry does not have time I would be happy to mail a
> revert to cd544fd1dc92 as we discussed in [1]. Dmitry, would you like
> me to do that?

Ack. I was planning to send a patches set that includes the revert, but
that's stalled a bit. As the patch just adds excessive checks, but
doesn't introduce an issue, I haven't sent it separately.
Feel free to revert it :-)

Thanks,
          Dmitry