Re: [PATCH 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware with sub-page faults

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Nov 29, 2021 at 01:53:01PM -0800, Linus Torvalds wrote:
> On Mon, Nov 29, 2021 at 12:56 PM Catalin Marinas
> <catalin.marinas@arm.com> wrote:
> > I think that would be useful, though it doesn't solve the potential
> > livelock with sub-page faults.
> 
> I was assuming we'd just do the sub-page faults.
> 
> In fact, I was assuming we'd basically just replace all the PAGE_ALIGN
> and PAGE_SIZE with SUBPAGE_{ALIGN,SIZE}, together with something like
> 
>         if (size > PAGE_SIZE)
>                 size = PAGE_SIZE;
> 
> to limit that size thing (or possibly make that "min size" be a
> parameter, so that people who have things like that "I need at least
> this initial structure to be copied" issue can document their minimum
> size needs).

Ah, so fault_in_writeable() would never fault in the whole range (if too
large). When copy_to_user() goes beyond the faulted in range, it may
fail and we go back to fault in a bit more of the range. A copy loop
would be equivalent to:

	fault_addr = ubuf;
	end = ubuf + size;
	while (1) {
		if (fault_in_writeable(fault_addr,
				       min(PAGE_SIZE, end - fault_addr)))
			break;
		left = copy_to_user(ubuf, kbuf, size);
		if (!left)
			break;
		fault_addr = end - left;
	}

That should work. I'll think about it tomorrow, getting late over here.

(I may still keep the sub-page probing in the arch code, see my earlier
exchanges with Andreas)

-- 
Catalin