On Thu, Nov 22, 2018 at 04:35:58PM +0100, Vlastimil Babka wrote: > >> I think I don't understand this comment :( Do you want to avoid waking > >> up kswapd from steal_suitable_fallback() (introduced above) for > >> allocations without __GFP_KSWAPD_RECLAIM? But returning 0 here means > >> actually allowing the allocation go through steal_suitable_fallback()? > >> So should it return ALLOC_NOFRAGMENT below, or was the intent different? > >> > > > > I want to avoid waking kswapd in steal_suitable_fallback if waking > > kswapd is not allowed. > > OK, but then this 'if' should return ALLOC_NOFRAGMENT, not 0? > But that will still not prevent waking kswapd for nodes where there's no > ZONE_DMA32, or any node when get_page_from_freelist() retries without > fallback. > > > If the calling context does not allow it, it does > > mean that fragmentation will be allowed to occur. I'm banking on it > > being a relatively rare case but potentially it'll be problematic. The > > main source of allocation requests that I expect to hit this are THP and > > as they are already at pageblock_order, it has limited impact from a > > fragmentation perspective -- particularly as pageblock_order stealing is > > allowed even with ALLOC_NOFRAGMENT. > > Yep, THP will skip the wakeup in steal_suitable_fallback() via 'goto > single_page' above it. For other users of ~__GFP_KSWAPD_RECLAIM (are > there any?) we could maybe just ignore and wakeup kswapd anyway, since > avoiding fragmentation is more important? Or if we wanted to avoid > wakeup reliably, then steal_suitable_fallback() would have to know and > check gfp_flags I'm afraid, and that doesn't seem worth the trouble. Indeed. While it works in some cases, it'll be full of holes and while I could close them, it just turns into a subtle mess. I've prepared a preparation path that encodes __GFP_KSWAPD_RECLAIM in alloc_flags and checks based on that. It's a lot cleaner overall, it's less of a mess than passing gfp_flags all the way through for one test and there are fewer side-effects. Thanks! -- Mel Gorman SUSE Labs
