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

[Vlastimil Babka <vbabka@suse.cz>]

On 5/4/21 10:06 PM, Shakeel Butt wrote:
> On Tue, May 4, 2021 at 1:02 PM Waiman Long <llong@redhat.com> wrote:
>>
>> On 5/4/21 3:37 PM, Shakeel Butt wrote:
>> > On Tue, May 4, 2021 at 6:24 AM Waiman Long <longman@redhat.com> 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@redhat.com>
>> >> ---
>> >>   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.
>>
> 
> No this is fine and DMA32 question is unrelated to this patch series.

We never supported them in kmalloc(), only explicit caches with SLAB_CACHE_DMA32
flag.