[RFC][PATCHSET] VM_FAULT_RETRY fixes

[Al Viro]

On Tue, Jan 31, 2023 at 12:24:54PM -0800, Linus Torvalds wrote:

> But *if* the alpha code were to just translate it into the
> FAULT_FLAG_xyz namespace, apretty much *all* of the alpha
> do_page_fault() could have been then done by a completely generic
> "generic_page_fault()" that has all of the retry logic, all of the
> si_code logic, etc etc.

Umm...  What about the semantics of get_user() of unmapped address?
Some architectures do quiet EFAULT; some (including alpha) hit
the sucker with SIGBUS, no matter what.

Examples:
arm
        if (likely(!(fault & (VM_FAULT_ERROR | VM_FAULT_BADMAP | VM_FAULT_BADACCESS))))
		return 0;

	/*
	 * If we are in kernel mode at this point, we
	 * have no context to handle this fault with.
	 */
	if (!user_mode(regs))
		goto no_context;
	...
alpha
 do_sigbus:
        mmap_read_unlock(mm);
        /* Send a sigbus, regardless of whether we were in kernel
           or user mode.  */
        force_sig_fault(SIGBUS, BUS_ADRERR, (void __user *) address);
        if (!user_mode(regs))
                goto no_context;
        return;

Are we free to modify that behaviour, or is that part of arch-specific
ABI?