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
