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

[Shakeel Butt <shakeelb@google.com>]

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.

> +
>  int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
>                                  gfp_t gfp, bool new_page)
>  {
> @@ -2870,6 +2877,7 @@ int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
>         unsigned long memcg_data;
>         void *vec;
>
> +       gfp &= ~OBJCGS_CLEAR_MASK;
>         vec = kcalloc_node(objects, sizeof(struct obj_cgroup *), gfp,
>                            page_to_nid(page));
>         if (!vec)
> diff --git a/mm/slab.h b/mm/slab.h
> index 18c1927cd196..b3294712a686 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -309,7 +309,6 @@ static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
>         if (!memcg_kmem_enabled() || !objcg)
>                 return;
>
> -       flags &= ~__GFP_ACCOUNT;
>         for (i = 0; i < size; i++) {
>                 if (likely(p[i])) {
>                         page = virt_to_head_page(p[i]);
> --
> 2.18.1
>