Re: [PATCH v2 2/2] mm: adds NOSIGBUS extension to mmap()

On Fri, Jun 04, 2021 at 12:43:22AM -0700, Ming Lin wrote:

Adds new flag MAP_NOSIGBUS of mmap() to specify the behavior of
"don't SIGBUS on fault". Right now, this flag is only allowed
for private mapping.

That's not what your use case asks for.

SIGBUS can be generated for a number of reasons, not only on fault beyond
end-of-file. vmf_error() would convert any errno, except ENOMEM to
VM_FAULT_SIGBUS.

Do you want to ignore -EIO or -ENOSPC? I don't think so.

For MAP_NOSIGBUS mapping, map in the zero page on read fault
or fill a freshly allocated page with zeroes on write fault.

I don't like the resulting semantics: if you had a read fault beyond EOF
and got zero page, you will still see zero page even if the file grows.
Yes, it's allowed by POSIX for MAP_PRIVATE to get out-of-sync with the
file, but it's not what users used to.

It might be enough for the use case, but I would rather avoid one-user
features.

-- 
 Kirill A. Shutemov