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@xxxxxxxxxxx> > > 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@xxxxxxx> Thanks!
