Re: [PATCH v2 1/2] mm: memcg/slab: Properly set up gfp flags for objcg pointer array

[Waiman Long]

On 5/4/21 3:37 PM, Shakeel Butt wrote:
> On Tue, May 4, 2021 at 6:24 AM Waiman Long <longman@xxxxxxxxxx> wrote:
> > Since the merging of the new slab memory controller in v5.9, the page
> > structure may store a pointer to obj_cgroup pointer array for slab pages.
> > Currently, only the __GFP_ACCOUNT bit is masked off. However, the array
> > is not readily reclaimable and doesn't need to come from the DMA buffer.
> > So those GFP bits should be masked off as well.
> >
> > Do the flag bit clearing at memcg_alloc_page_obj_cgroups() to make sure
> > that it is consistently applied no matter where it is called.
> >
> > Fixes: 286e04b8ed7a ("mm: memcg/slab: allocate obj_cgroups for non-root slab pages")
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   mm/memcontrol.c | 8 ++++++++
> >   mm/slab.h       | 1 -
> >   2 files changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index c100265dc393..5e3b4f23b830 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2863,6 +2863,13 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
> >   }
> >
> >   #ifdef CONFIG_MEMCG_KMEM
> > +/*
> > + * The allocated objcg pointers array is not accounted directly.
> > + * Moreover, it should not come from DMA buffer and is not readily
> > + * reclaimable. So those GFP bits should be masked off.
> > + */
> > +#define OBJCGS_CLEAR_MASK      (__GFP_DMA | __GFP_RECLAIMABLE | __GFP_ACCOUNT)
> What about __GFP_DMA32? Does it matter? It seems like DMA32 requests
> go to normal caches.

I included __GFP_DMA32 in my first draft patch. However, __GFP_DMA32 is 
not considered in determining the right kmalloc_type() (patch 2), so I 
took it out to make it consistent. I can certainly add it back.

Cheers,
Longman