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    SubjectRe: [PATCH v2 02/11] memcg: Reclaim when more than one page needed.
    On Thu 09-08-12 17:01:10, Glauber Costa wrote:
    [...]
    > @@ -2317,18 +2318,18 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
    > } else
    > mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
    > /*
    > - * nr_pages can be either a huge page (HPAGE_PMD_NR), a batch
    > - * of regular pages (CHARGE_BATCH), or a single regular page (1).
    > - *
    > * Never reclaim on behalf of optional batching, retry with a
    > * single page instead.
    > */
    > - if (nr_pages == CHARGE_BATCH)
    > + if (nr_pages > min_pages)
    > return CHARGE_RETRY;

    This is dangerous because THP charges will be retried now while they
    previously failed with CHARGE_NOMEM which means that we will keep
    attempting potentially endlessly.
    Why cannot we simply do if (nr_pages < CHARGE_BATCH) and get rid of the
    min_pages altogether?
    Also the comment doesn't seem to be valid anymore.

    >
    > if (!(gfp_mask & __GFP_WAIT))
    > return CHARGE_WOULDBLOCK;
    >
    > + if (gfp_mask & __GFP_NORETRY)
    > + return CHARGE_NOMEM;
    > +
    > ret = mem_cgroup_reclaim(mem_over_limit, gfp_mask, flags);
    > if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
    > return CHARGE_RETRY;
    > @@ -2341,7 +2342,7 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
    > * unlikely to succeed so close to the limit, and we fall back
    > * to regular pages anyway in case of failure.
    > */
    > - if (nr_pages == 1 && ret)
    > + if (nr_pages <= (1 << PAGE_ALLOC_COSTLY_ORDER) && ret)
    > return CHARGE_RETRY;
    >
    > /*
    > @@ -2476,7 +2477,8 @@ again:
    > nr_oom_retries = MEM_CGROUP_RECLAIM_RETRIES;
    > }
    >
    > - ret = mem_cgroup_do_charge(memcg, gfp_mask, batch, oom_check);
    > + ret = mem_cgroup_do_charge(memcg, gfp_mask, batch, nr_pages,
    > + oom_check);
    > switch (ret) {
    > case CHARGE_OK:
    > break;
    > --
    > 1.7.11.2
    >
    > --
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    --
    Michal Hocko
    SUSE Labs


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