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

From: Catalin Marinas <[email-masked]>
Date: Mon, 29 Nov 2021 20:56:08 +0000

On Mon, Nov 29, 2021 at 10:40:38AM -0800, Linus Torvalds wrote:
> On Mon, Nov 29, 2021 at 7:36 AM Catalin Marinas <[email-masked]> wrote:
> > That's what this series does when it probes the whole range in
> > fault_in_writeable(). The main reason was that it's more efficient to do
> > a read than a write on a large range (the latter dirtying the cache
> > lines).
> 
> The more this thread goes on, the more I'm starting to think that we
> should just make "fault_in_writable()" (and readable, of course) only
> really work on the beginning of the area.
> 
> Not just for the finer-granularity pointer color probing, but for the
> page probing too.

I have patches for the finer-granularity checking of the beginning of
the buffer. They need a bit of testing, so probably posting them
tomorrow.

> I'm looking at our current fault_in_writeable(), and I'm going
> 
>  (a) it uses __put_user() without range checks, which is really not great

For arm64 at least __put_user() does the access_ok() check. I thought
only unsafe_put_user() should skip the checks. If __put_user() can write
arbitrary memory, we may have a bigger problem.

>  (b) it looks like a disaster from another standpoint: essentially
> user-controlled loop size with no limit checking, no preemption, and
> no check for fatal signals.

Indeed, the fault_in_*() loop can get pretty long, bounded by how much
memory can be faulted in the user process. My patches for now only
address the outer loop doing the copy_to_user() as that can be
unbounded.

> Now, (a) should be fixed with a access_ok() or similar.
> 
> And (b) can easily be fixed multiple ways, with one option simply just
> being adding a can_resched() call and checking for fatal signals.
> 
> But faulting in the whole region is actually fundamentally wrong in
> low-memory situations - the beginning of the region might be swapped
> out by the time we get to the end. That's unlikely to be a problem in
> real life, but it's an example of how it's simply not conceptually
> sensible.
> 
> So I do wonder why we don't just say "fault_in_writable will fault in
> _at_most_ X bytes", and simply limit the actual fault-in size to
> something reasonable.
> 
> That solves _all_ the problems. It solves the lack of preemption and
> fatal signals (by virtue of just limiting the amount of work we do).
> It solves the low memory situation. And it solves the "excessive dirty
> cachelines" case too.

I think that would be useful, though it doesn't solve the potential
livelock with sub-page faults. We still need the outer loop to
handle the copy_to_user() for the whole user buffer and the sub-page
probing of the beginning of such buffer (or whenever copy_to_user()
failed). IOW, you still fault in the whole buffer eventually.

Anyway, I think the sub-page probing and limiting the fault-in are
complementary improvements.

-- 
Catalin

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