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    SubjectRe: [PATCH v2 2/2] mm/memcg: set memcg when split page
    On Tue 09-03-21 12:32:55, Matthew Wilcox wrote:
    > On Tue, Mar 09, 2021 at 10:02:00AM +0100, Michal Hocko wrote:
    [...]
    > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > > index 913c2b9e5c72..d44dea2b8d22 100644
    > > --- a/mm/memcontrol.c
    > > +++ b/mm/memcontrol.c
    > > @@ -3135,8 +3135,21 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
    > > if (memcg && !mem_cgroup_is_root(memcg)) {
    > > ret = __memcg_kmem_charge(memcg, gfp, 1 << order);
    > > if (!ret) {
    > > + int nr_pages = 1 << order;
    > > page->memcg_data = (unsigned long)memcg |
    > > MEMCG_DATA_KMEM;
    > > +
    > > + /*
    > > + * Compound pages are normally split or freed
    > > + * via their head pages so memcg_data in in the
    > > + * head page should be sufficient but there
    > > + * are exceptions to the rule (see __free_pages).
    > > + * Non compound pages would need to copy memcg anyway.
    > > + */
    > > + for (i = 1; i < nr_pages; i++) {
    > > + struct page * p = page + i;
    > > + p->memcg_data = page->memcg_data
    > > + }
    > > return 0;
    >
    > I would condition this loop on if (!(gfp & __GFP_COMP)), but yes, something
    > along these lines. I might phrase the comment a little differently ...
    >
    > /*
    > * Compound pages are treated as a single unit,
    > * but non-compound pages can be freed individually
    > * so each page needs to have its memcg set to get
    > * the accounting right.
    > */

    OK, I must have misunderstood your __free_pages fix then. I thought this
    was about compound pages. Btw. again I forgot about css ref counting so
    here is an updated version.

    diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    index 913c2b9e5c72..ec2c705f38fa 100644
    --- a/mm/memcontrol.c
    +++ b/mm/memcontrol.c
    @@ -3133,10 +3133,22 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)

    memcg = get_mem_cgroup_from_current();
    if (memcg && !mem_cgroup_is_root(memcg)) {
    - ret = __memcg_kmem_charge(memcg, gfp, 1 << order);
    + int nr_pages = 1 << order;
    + ret = __memcg_kmem_charge(memcg, gfp, nr_pages);
    if (!ret) {
    page->memcg_data = (unsigned long)memcg |
    MEMCG_DATA_KMEM;
    + if (nr_pages > 1) {
    + /*
    + * comment goes here
    + */
    + for (i = 1; i < nr_pages; i++) {
    + struct page * p = page + i;
    + p->memcg_data = page->memcg_data
    + }
    + /* Head page reference from get_mem_cgroup_from_current */
    + css_get_many(&memcg->css, nr_pages - 1);
    + }
    return 0;
    }
    css_put(&memcg->css);
    --
    Michal Hocko
    SUSE Labs

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