On Tue 26-01-21 11:10:18, Minchan Kim wrote: > On Tue, Jan 26, 2021 at 08:44:49AM +0100, Michal Hocko wrote: > > On Mon 25-01-21 11:33:36, Minchan Kim wrote: > > > On Mon, Jan 25, 2021 at 02:12:00PM +0100, Michal Hocko wrote: > > > > On Thu 21-01-21 09:55:00, Minchan Kim wrote: > > > > > Contiguous memory allocation can be stalled due to waiting > > > > > on page writeback and/or page lock which causes unpredictable > > > > > delay. It's a unavoidable cost for the requestor to get *big* > > > > > contiguous memory but it's expensive for *small* contiguous > > > > > memory(e.g., order-4) because caller could retry the request > > > > > in different range where would have easy migratable pages > > > > > without stalling. > > > > > > > > > > This patch introduce __GFP_NORETRY as compaction gfp_mask in > > > > > alloc_contig_range so it will fail fast without blocking > > > > > when it encounters pages needed waiting. > > > > > > > > I am not against controling how hard this allocator tries with gfp mask > > > > but this changelog is rather void on any data and any user. > > > > > > > > It is also rather dubious to have retries when then caller says to not > > > > retry. > > > > > > Since max_tries is 1 with ++tries, it shouldn't retry. > > > > OK, I have missed that. This is a tricky code. ASYNC mode should be > > completely orthogonal to the retries count. Those are different things. > > Page allocator does an explicit bail out based on __GFP_NORETRY. You > > should be doing the same. > > A concern with __GFP_NOWAIT is regardless of flags passed to cma_alloc, > internal implementation of alloc_contig_range inside will use blockable > operation. See __alloc_contig_migrate_range. Yes it is now. But nothing should prevent from making it non blockable. > If we go with __GFP_NOWAIT, we should propagate the gfp_mask inside of > __alloc_contig_migrate_range to make cma_alloc consistent with alloc_pages. Absolutely. You should be doing that anyway. As I've said above you shouldn't rely on side effects like ASYNC mode. > (IIUC, that's what you want - make gfp_mask consistent between cma_alloc > and alloc_pages) but I am worry about the direction will make complicate > situation since cma invovles migration context as well as target page > allocation context. Sometime, the single gfp flag could be trouble > to express both contexts all at once. I am not sure I see your concern. > > > > Also why didn't you consider GFP_NOWAIT semantic for non blocking mode? > > > > > > GFP_NOWAIT seems to be low(specific) flags rather than the one I want to > > > express. Even though I said only page writeback/lock in the description, > > > the goal is to avoid costly operations we might find later so such > > > "failfast", I thought GFP_NORETRY would be good fit. > > > > I suspect you are too focused on implementation details here. Think > > about the indended semantic. Callers of this functionality will not > > think about those (I hope because if they rely on these details then the > > whole thing will become unmaintainable because any change would require > > an audit of all existing users). All you should be caring about is to > > control how expensive the call can be. GFP_NOWAIT is not really low > > level from that POV. It gives you a very lightweight non-sleeping > > attempt to allocate. GFP_NORETRY will give you potentially sleeping but > > an opportunistic-easy-to-fail attempt. And so on. See how that is > > absolutely free of any page writeback or any specific locking. > > With above reason I mentioned, I wanted to express __GFP_NORETRY as > "opportunistic-easy-to-fail attempt" to support cma_alloc as "failfast" > for migration context. Yes that is fine. And please note that I do not push for NOWAIT semantic. If there is no user for that now then fine. -- Michal Hocko SUSE Labs
