Re: [PATCH linux-next] mm: swap: get rid of deadloop in swapin readahead

[Andrew Morton]

On Mon, 28 Feb 2022 20:07:33 -0800 (PST) Hugh Dickins <hughd@xxxxxxxxxx> wrote:

> > > --- a/mm/swap_state.c
> > > +++ b/mm/swap_state.c
> > > @@ -478,7 +478,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> > >  		 * __read_swap_cache_async(), which has set SWAP_HAS_CACHE
> > >  		 * in swap_map, but not yet added its page to swap cache.
> > >  		 */
> > > -		cond_resched();
> > > +		schedule_timeout_uninterruptible(1);
> > >  	}
> > >  
> > >  	/*
> > 
> > Sigh.  I guess yes, we should do this, at least in a short-term,
> > backportable-to-stable way.
> > 
> > But busy-waiting while hoping that someone else will save us isn't an
> > attractive design.  Hugh, have you ever thought about something more
> > deterministic in there?
> 
> Not something more deterministic, no: I think that would entail
> heavier locking, perhaps slowing down hotter paths, just to avoid
> this swap oddity.
> 
> This loop was written long before there was a preemptive kernel:
> it was appropriate then, and almost never needed more than one retry
> to complete; but preemption changed the story without us realizing.
> 
> Sigh here too.  I commend the thread on it from July 2018:
> https://lore.kernel.org/linux-mm/2018072514403228778860@xxxxxxxxxxxx/
> 
> There the 4.9-stable user proposed preempt_disable(), I agreed but
> found the patch provided insufficient, and offered another 4.9 patch
> further down the thread.  Your preference at the time was msleep(1).
> 
> I was working on a similar patch for 4.18, but have not completed it
> yet ;) and don't remember how satisfied or not I was with that one;
> and wonder if I'm any more likely to get it finished by 2026.  It's
> clear that I put much more thought into it back then than just now.
> 
> Maybe someone else would have a go: my 4.9 patch in that thread
> shows most of it, but might need a lot of work to update to 5.17.
> 
> And it also gathered some temporary debug stats on how often this
> happens: I'm not conscious of using RT at all, but was disturbed to see
> how long an ordinary preemptive kernel was sometimes spinning there.
> So I think I agree with you more than Michal on that: RT just makes
> the bad behaviour more obvious.

Thanks as always.

Using msleep() seems pretty pointless so I plan to go ahead with patch
as-is, with a cc:stable.  None of it is pretty, but it's better than
what we have now, yes?