Re: [PATCH] mm: compaction: avoid GFP_NOFS ABBA deadlock

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, May 24, 2023 at 11:21:43AM +0200, Vlastimil Babka wrote:
> On 5/19/23 13:13, Johannes Weiner wrote:
> > During stress testing with higher-order allocations, a deadlock
> > scenario was observed in compaction: One GFP_NOFS allocation was
> > sleeping on mm/compaction.c::too_many_isolated(), while all CPUs in
> > the system were busy with compactors spinning on buffer locks held by
> > the sleeping GFP_NOFS allocation.
> > 
> > Reclaim is susceptible to this same deadlock; we fixed it by granting
> > GFP_NOFS allocations additional LRU isolation headroom, to ensure it
> > makes forward progress while holding fs locks that other reclaimers
> > might acquire. Do the same here.
> > 
> > This code has been like this since compaction was initially merged,
> > and I only managed to trigger this with out-of-tree patches that
> > dramatically increase the contexts that do GFP_NOFS compaction. While
> > the issue is real, it seems theoretical in nature given existing
> > allocation sites. Worth fixing now, but no Fixes tag or stable CC.
> 
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> 
> So IIUC the change is done by not giving GFP_NOFS extra headroom, but
> instead restricting the headroom of __GFP_FS allocations. But the original
> one was probably too generous anyway so it should be fine?

Yes, the original limit is generally half the LRU, which is quite high.

The new limit is 1/16th of the LRU for regular compactors and half for
GFP_NOFS ones. Note that I didn't make these up; they're stolen from
too_many_isolated() in vmscan.c. I figured those are proven values and
no sense in deviating from them until we have a reason to do so.

> Acked-by: Vlastimil Babka <vbabka@suse.cz>

Thanks!