Re: untagged_addr_remote() in do_madvise()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Tue, Jan 14, 2025 at 01:13:36PM -0800, Dave Hansen wrote:
> On 1/14/25 12:41, Lorenzo Stoakes wrote:
> ...
> > However, MADV_HWPOISON, MADV_SOFT_OFFLINE seems fundamentally broken for tagged
> > addresses:
> >
> > #ifdef CONFIG_MEMORY_FAILURE
> > 	if (behavior == MADV_HWPOISON || behavior == MADV_SOFT_OFFLINE)
> > 		return madvise_inject_error(behavior, start, start + len_in);
> > #endif
> >
> > ^ this is invoked before untagged_addr_remote() is called (as no mmap lock is
> > acquired) and so no attempt at untagging happens at all...!
>
> Except this call path:
>
> 	madvise_inject_error() ->
> 	get_user_pages_fast() ->
> 	gup_fast_fallback()
>
> does its own untagging:
>
>         start = untagged_addr(start) & PAGE_MASK;
>

Yeah you're right! Good spot.

> It might also have some funky behavior if start+len_in overflows. But,
> just as in the other case, it's invalid to begin with so I think
> userspace kinda gets to keep the pieces.

Right yeah.

>
> But I do 100% agree that this is non-obvious. In a perfect world, tagged
> addresses would get untagged at the user/kernel boundary in _one_ choke
> point. But the world is hard and that would make things too easy and
> then we wouldn't get paid the big bucks. ;)

Yeah agreed especially on that last bit ;)

I think it'd be good to have a comment there, I will stick on my todo to
add one.

Or Liam - if you're doing some changes here - maybe you could add? Just
something highlighting that gup_fast does the untagging?

>
> To clarify things, I don't think it'd be the worst thing to just move
> the madvise_inject_error() down and have that case acquire
> mmap_read_lock(). Sure, it's not required, but it's basically debugging
> code and I can't imagine it's avoiding the lock for performance reasons.

Yeah it's odd that, but that code is going to the lengths of using gup_fast
so I have to assume that maybe some debug user really does care about perf?
It'd need some more digging to really feel confident to use a lock there I
think.