On Tue, Jul 07, 2020 at 01:40:19PM +0200, Michal Hocko wrote: > On Tue 07-07-20 16:44:43, Joonsoo Kim wrote: > > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> > > > > In mm/migrate.c, THP allocation for migration is called with the provided > > gfp_mask | GFP_TRANSHUGE. This gfp_mask contains __GFP_RECLAIM and it > > would be conflict with the intention of the GFP_TRANSHUGE. > > > > GFP_TRANSHUGE/GFP_TRANSHUGE_LIGHT is introduced to control the reclaim > > behaviour by well defined manner since overhead of THP allocation is > > quite large and the whole system could suffer from it. So, they deals > > with __GFP_RECLAIM mask deliberately. If gfp_mask contains __GFP_RECLAIM > > and uses gfp_mask | GFP_TRANSHUGE(_LIGHT) for THP allocation, it means > > that it breaks the purpose of the GFP_TRANSHUGE(_LIGHT). > > GFP_TRANSHUGE* is not a carved in stone design. Their primary reason to > exist is to control how hard to try for different allocation > paths/configurations because their latency expectations might be > largerly different. It is mostly the #PF path which aims to be as > lightweight as possible I believe nobody simply considered migration to be > very significant to even care. And I am still not sure it matters but > I would tend to agree that a consistency here is probably a very minor > plus. > > Your changelog is slightly misleading in that regard because it suggests > that this is a real problem while it doesn't present any actual data. > It would be really nice to make the effective change really stand out. > We are only talking about __GFP_RECLAIM_KSWAPD here. So the only > difference is that the migration won't wake up kswapd now. > > All that being said the changelog should be probably more explicit about > the fact that this is solely done for consistency and be honest that the > runtime effect is not really clear. This would help people reading it in > future. Okay. How about following changelog? Thanks. ----------->8-------------------- Subject: [PATCH] mm/migrate: clear __GFP_RECLAIM for THP allocation for migration In migration target allocation functions, THP allocations uses different gfp_mask, especially, in regard to the reclaim gfp_mask. There is no reason to use different reclaim gfp_mask for each cases and it is an obstacle to make a common function in order to clean-up migration target allocation functions. This patch fixes this situation by using common reclaim gfp_mask for THP allocation.
