On 08/28/2017 03:11 AM, [email protected] wrote: > From: Joonsoo Kim <[email protected]> > > High-order atomic allocation is difficult to succeed since we cannot > reclaim anything in this context. So, we reserves the pageblock for > this kind of request. > > In slub, we try to allocate higher-order page more than it actually > needs in order to get the best performance. If this optimistic try is > used with GFP_ATOMIC, alloc_flags will be set as ALLOC_HARDER and > the pageblock reserved for high-order atomic allocation would be used. > Moreover, this request would reserve the MIGRATE_HIGHATOMIC pageblock > ,if succeed, to prepare further request. It would not be good to use > MIGRATE_HIGHATOMIC pageblock in terms of fragmentation management > since it unconditionally set a migratetype to request's migratetype > when unreserving the pageblock without considering the migratetype of > used pages in the pageblock. > > This is not what we don't intend so fix it by unconditionally setting > __GFP_NOMEMALLOC in order to not set ALLOC_HARDER.
I wonder if it would be more robust to strip GFP_ATOMIC from alloc_gfp. E.g. __GFP_NOMEMALLOC does seem to prevent ALLOC_HARDER, but not ALLOC_HIGH. Or maybe we should adjust __GFP_NOMEMALLOC implementation and document it more thoroughly? CC Michal Hocko Also, were these 2 patches done via code inspection or you noticed suboptimal behavior which got fixed? Thanks. > Signed-off-by: Joonsoo Kim <[email protected]> > --- > mm/slub.c | 6 ++---- > 1 file changed, 2 insertions(+), 4 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index e1e442c..fd8dd89 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -1579,10 +1579,8 @@ static struct page *allocate_slab(struct kmem_cache > *s, gfp_t flags, int node) > */ > alloc_gfp = (flags | __GFP_NOWARN | __GFP_NORETRY) & ~__GFP_NOFAIL; > if (oo_order(oo) > oo_order(s->min)) { > - if (alloc_gfp & __GFP_DIRECT_RECLAIM) { > - alloc_gfp |= __GFP_NOMEMALLOC; > - alloc_gfp &= ~__GFP_DIRECT_RECLAIM; > - } > + alloc_gfp |= __GFP_NOMEMALLOC; > + alloc_gfp &= ~__GFP_DIRECT_RECLAIM; > } > > page = alloc_slab_page(s, alloc_gfp, node, oo); >
