On Tue, Dec 10, 2024 at 10:05:22AM +0100, Michal Hocko wrote: > On Tue 10-12-24 05:31:30, Matthew Wilcox wrote: > > On Mon, Dec 09, 2024 at 06:39:31PM -0800, Alexei Starovoitov wrote: > > > + if (preemptible() && !rcu_preempt_depth()) > > > + return alloc_pages_node_noprof(nid, > > > + GFP_NOWAIT | __GFP_ZERO, > > > + order); > > > + return alloc_pages_node_noprof(nid, > > > + __GFP_TRYLOCK | __GFP_NOWARN | __GFP_ZERO, > > > + order); > > > > [...] > > > > > @@ -4009,7 +4018,7 @@ gfp_to_alloc_flags(gfp_t gfp_mask, unsigned int order) > > > * set both ALLOC_NON_BLOCK and ALLOC_MIN_RESERVE(__GFP_HIGH). > > > */ > > > alloc_flags |= (__force int) > > > - (gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM)); > > > + (gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM | __GFP_TRYLOCK)); > > > > It's not quite clear to me that we need __GFP_TRYLOCK to implement this. > > I was originally wondering if this wasn't a memalloc_nolock_save() / > > memalloc_nolock_restore() situation (akin to memalloc_nofs_save/restore), > > but I wonder if we can simply do: > > > > if (!preemptible() || rcu_preempt_depth()) > > alloc_flags |= ALLOC_TRYLOCK; > > preemptible is unusable without CONFIG_PREEMPT_COUNT but I do agree that > __GFP_TRYLOCK is not really a preferred way to go forward. For 3 > reasons. > > First I do not really like the name as it tells what it does rather than > how it should be used. This is a general pattern of many gfp flags > unfotrunatelly and historically it has turned out error prone. If a gfp > flag is really needed then something like __GFP_ANY_CONTEXT should be > used. If the current implementation requires to use try_lock for > zone->lock or other changes is not an implementation detail but the user > should have a clear understanding that allocation is allowed from any > context (NMI, IRQ or otherwise atomic contexts). > > Is there any reason why GFP_ATOMIC cannot be extended to support new GFP_ATOMIC has access to memory reserves. I see GFP_NOWAIT a better fit and if someone wants access to the reserve they can use __GFP_HIGH with GFP_NOWAIT. > contexts? This allocation mode is already documented to be usable from > atomic contexts except from NMI and raw_spinlocks. But is it feasible to > extend the current implementation to use only trylock on zone->lock if > called from in_nmi() to reduce unexpected failures on contention for > existing users? I think this is the question we (MM folks) need to answer, not the users. > > Third, do we even want such a strong guarantee in the generic page > allocator path and make it even more complex and harder to maintain? I think the alternative would be higher maintenance cost i.e. everyone creating their own layer/solution/caching over page allocator which I think we agree we want to avoid (Vlastimil's LSFMM talk). > We > already have a precence in form of __alloc_pages_bulk which is a special > case allocator mode living outside of the page allocator path. It seems > that it covers most of your requirements except the fallback to the > regular allocation path AFAICS. Is this something you could piggy back > on? BPF already have bpf_mem_alloc() and IIUC this series is an effort to unify and have a single solution.
