Re: [PATCH 2/2] mm: adds NOSIGBUS extension for out-of-band shmem read

[Hugh Dickins]

On Wed, 2 Jun 2021, Ming Lin wrote:
> 
> This is what I wrote so far.
> 
> ---
>  include/linux/mm.h                     |  2 ++
>  include/linux/mman.h                   |  1 +
>  include/uapi/asm-generic/mman-common.h |  1 +
>  mm/memory.c                            | 12 ++++++++++++
>  mm/mmap.c                              |  4 ++++
>  5 files changed, 20 insertions(+)

I have not looked at the rest, just looking at mm/memory.c:

> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3676,6 +3676,18 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
>  	}
>   	ret = vma->vm_ops->fault(vmf);
> +	if (unlikely(ret & VM_FAULT_SIGBUS) && (vma->vm_flags & VM_NOSIGBUS))
> {
> +		/*
> +		 * Get zero page for MAP_NOSIGBUS mapping, which isn't
> +		 * coherent wrt shmem contents that are expanded and
> +		 * filled in later.
> +		 */
> +		vma->vm_flags |= VM_MIXEDMAP;
> +		if (!vm_insert_page(vma, (unsigned long)vmf->address,
> +				ZERO_PAGE(vmf->address)))
> +			return VM_FAULT_NOPAGE;
> +	}
> +
>  	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY
> |
>  			    VM_FAULT_DONE_COW)))
>  		return ret;

Sorry, I directed you to mm/memory.c without indicating what's
appropriate here.  Please don't attempt to use VM_MIXEDMAP and
vm_insert_page(): they're for special driver mmaps, they're no
better here than they were in mm/shmem.c.

It's do_anonymous_page()'s business to map in the zero page on
read fault (see "my_zero_pfn(vmf->address)" in there), or fill
a freshly allocated page with zeroes on write fault - and now
you're sticking to MAP_PRIVATE, write faults in VM_WRITE areas
are okay for VM_NOSIGBUS.

Ideally you can simply call do_anonymous_page() from __do_fault()
in the VM_FAULT_SIGBUS on VM_NOSIGBUS case.  That's what to start
from anyway: but look to see if there's state to be adjusted to
achieve that; and it won't be surprising if somewhere down in
do_anonymous_page() or something it calls, there's a BUG on it
being called when vma->vm_file is set, or something like that.
May need some tweaking.

Hugh