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

[Andreas Gruenbacher]

On Mon, Nov 29, 2021 at 7:41 PM Linus Torvalds
<torvalds at linux-foundation.org> wrote:
> On Mon, Nov 29, 2021 at 7:36 AM Catalin Marinas <catalin.marinas at arm.com> 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'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
>
>  (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.
>
> 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.
>
> Of course, we want to have some minimum bytes we fault in too, but
> that minimum range might well be "we guarantee at least a full page
> worth of data" (and in practice make it a couple of pages).
>
> It's not like fault_in_writeable() avoids page faults or anything like
> that - it just moves them around. So there's really very little reason
> to fault in a large range, and there are multiple reasons _not_ to do
> it.
>
> Hmm?

This would mean that we could get rid of gfs2's
should_fault_in_pages() logic, which is based on what's in
btrfs_buffered_write().

Andreas

>
>                Linus
>