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

[Brian Geffon]

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?

> Was there an mremap(2) man page update for MREMAP_DONTUNMAP?
> Whether or not there was before, it ought to get one now.

Yes, the mremap(2) man page was updated when this flag was added and
it will require a further update to reflect this expanded mapping
support.

Thanks
Brian

1. https://lkml.org/lkml/2020/12/28/2340