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Date: Thu, 10 Dec 2015 11:00:27 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Vladimir Davydov <vdavydov@...tuozzo.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] mm: memcontrol: charge swap to cgroup2
On Thu, Dec 10, 2015 at 02:39:14PM +0300, Vladimir Davydov wrote:
> In the legacy hierarchy we charge memsw, which is dubious, because:
>
> - memsw.limit must be >= memory.limit, so it is impossible to limit
> swap usage less than memory usage. Taking into account the fact that
> the primary limiting mechanism in the unified hierarchy is
> memory.high while memory.limit is either left unset or set to a very
> large value, moving memsw.limit knob to the unified hierarchy would
> effectively make it impossible to limit swap usage according to the
> user preference.
>
> - memsw.usage != memory.usage + swap.usage, because a page occupying
> both swap entry and a swap cache page is charged only once to memsw
> counter. As a result, it is possible to effectively eat up to
> memory.limit of memory pages *and* memsw.limit of swap entries, which
> looks unexpected.
>
> That said, we should provide a different swap limiting mechanism for
> cgroup2.
>
> This patch adds mem_cgroup->swap counter, which charges the actual
> number of swap entries used by a cgroup. It is only charged in the
> unified hierarchy, while the legacy hierarchy memsw logic is left
> intact.
>
> The swap usage can be monitored using new memory.swap.current file and
> limited using memory.swap.max.
>
> Signed-off-by: Vladimir Davydov <vdavydov@...tuozzo.com>
This looks great!
Acked-by: Johannes Weiner <hannes@...xchg.org>
I have a few questions, but none of them show-stoppers:
> ---
> include/linux/memcontrol.h | 1 +
> include/linux/swap.h | 5 ++
> mm/memcontrol.c | 123 +++++++++++++++++++++++++++++++++++++++++----
> mm/shmem.c | 4 ++
> mm/swap_state.c | 5 ++
> 5 files changed, 129 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index c6a5ed2f2744..993c9a26b637 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -169,6 +169,7 @@ struct mem_cgroup {
>
> /* Accounted resources */
> struct page_counter memory;
> + struct page_counter swap;
> struct page_counter memsw;
> struct page_counter kmem;
We should probably separate this to differentiate the new counters
from the old ones. Only memory and swap are actual resources, the
memsw and kmem counters are counting consumer-oriented.
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 457181844b6e..f4b3ccdcba91 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -368,11 +368,16 @@ static inline int mem_cgroup_swappiness(struct mem_cgroup *mem)
> #endif
> #ifdef CONFIG_MEMCG_SWAP
> extern void mem_cgroup_swapout(struct page *page, swp_entry_t entry);
> +extern int mem_cgroup_charge_swap(struct page *page, swp_entry_t entry);
Should this be mem_cgroup_try_swap() to keep in line with the page
counter terminology? So it's clear this is not forcing a charge.
> @@ -1248,12 +1248,15 @@ static unsigned long mem_cgroup_get_limit(struct mem_cgroup *memcg)
> {
> unsigned long limit;
>
> - limit = memcg->memory.limit;
> + limit = READ_ONCE(memcg->memory.limit);
> if (mem_cgroup_swappiness(memcg)) {
> unsigned long memsw_limit;
> + unsigned long swap_limit;
>
> - memsw_limit = memcg->memsw.limit;
> - limit = min(limit + total_swap_pages, memsw_limit);
> + memsw_limit = READ_ONCE(memcg->memsw.limit);
> + swap_limit = min(READ_ONCE(memcg->swap.limit),
> + (unsigned long)total_swap_pages);
> + limit = min(limit + swap_limit, memsw_limit);
> }
> return limit;
This is taking a racy snapshot, so we don't rely on 100% accuracy. Can
we do without the READ_ONCE()?
> @@ -5754,26 +5760,66 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
> memcg_check_events(memcg, page);
> }
>
> +/*
> + * mem_cgroup_charge_swap - charge a swap entry
> + * @page: page being added to swap
> + * @entry: swap entry to charge
> + *
> + * Try to charge @entry to the memcg that @page belongs to.
> + *
> + * Returns 0 on success, -ENOMEM on failure.
> + */
> +int mem_cgroup_charge_swap(struct page *page, swp_entry_t entry)
> +{
> + struct mem_cgroup *memcg;
> + struct page_counter *counter;
> + unsigned short oldid;
> +
> + if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) || !do_swap_account)
> + return 0;
> +
> + memcg = page->mem_cgroup;
> +
> + /* Readahead page, never charged */
> + if (!memcg)
> + return 0;
> +
> + if (!mem_cgroup_is_root(memcg) &&
> + !page_counter_try_charge(&memcg->swap, 1, &counter))
> + return -ENOMEM;
> +
> + oldid = swap_cgroup_record(entry, mem_cgroup_id(memcg));
> + VM_BUG_ON_PAGE(oldid, page);
> + mem_cgroup_swap_statistics(memcg, true);
> +
> + css_get(&memcg->css);
I think we don't have to duplicate the swap record code. Both cgroup1
and cgroup2 could run this function to handle the swapout record and
statistics, and then mem_cgroup_swapout() would simply uncharge memsw.
> @@ -5828,6 +5931,8 @@ static int __init mem_cgroup_swap_init(void)
> {
> if (!mem_cgroup_disabled() && really_do_swap_account) {
> do_swap_account = 1;
> + WARN_ON(cgroup_add_dfl_cftypes(&memory_cgrp_subsys,
> + swap_files));
> WARN_ON(cgroup_add_legacy_cftypes(&memory_cgrp_subsys,
> memsw_cgroup_files));
I guess we could also support cgroup.memory=noswap.
--
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