Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:
> On Mon, Aug 21, 2023 at 6:54 PM Liu Shixin <liushixin2@xxxxxxxxxx> wrote:
>>
>> When spaces of swap devices are exhausted, only file pages can be reclaimed.
>> But there are still some swapcache pages in anon lru list. This can lead
>> to a premature out-of-memory.
>>
>> This problem can be fixed by checking number of swapcache pages in
>> can_reclaim_anon_pages(). For memcg v2, there are swapcache stat that can
>> be used directly. For memcg v1, use total_swapcache_pages() instead, which
>> may not accurate but can solve the problem.
>
> Interesting find. I wonder if we really don't have any handling of
> this situation.
>
>>
>> Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
>> ---
>> include/linux/swap.h | 6 ++++++
>> mm/memcontrol.c | 8 ++++++++
>> mm/vmscan.c | 12 ++++++++----
>> 3 files changed, 22 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 456546443f1f..0318e918bfa4 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -669,6 +669,7 @@ static inline void mem_cgroup_uncharge_swap(swp_entry_t entry, unsigned int nr_p
>> }
>>
>> extern long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg);
>> +extern long mem_cgroup_get_nr_swapcache_pages(struct mem_cgroup *memcg);
>> extern bool mem_cgroup_swap_full(struct folio *folio);
>> #else
>> static inline void mem_cgroup_swapout(struct folio *folio, swp_entry_t entry)
>> @@ -691,6 +692,11 @@ static inline long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg)
>> return get_nr_swap_pages();
>> }
>>
>> +static inline long mem_cgroup_get_nr_swapcache_pages(struct mem_cgroup *memcg)
>> +{
>> + return total_swapcache_pages();
>> +}
>> +
>> static inline bool mem_cgroup_swap_full(struct folio *folio)
>> {
>> return vm_swap_full();
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index e8ca4bdcb03c..3e578f41023e 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -7567,6 +7567,14 @@ long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg)
>> return nr_swap_pages;
>> }
>>
>> +long mem_cgroup_get_nr_swapcache_pages(struct mem_cgroup *memcg)
>> +{
>> + if (mem_cgroup_disabled() || do_memsw_account())
>> + return total_swapcache_pages();
>> +
>> + return memcg_page_state(memcg, NR_SWAPCACHE);
>> +}
>
> Is there a reason why we cannot use NR_SWAPCACHE for cgroup v1? Isn't
> that being maintained regardless of cgroup version? It is not exposed
> in cgroup v1's memory.stat, but I don't think there is a reason we
> can't do that -- if only to document that it is being used with cgroup
> v1.
>
>
>> +
>> bool mem_cgroup_swap_full(struct folio *folio)
>> {
>> struct mem_cgroup *memcg;
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 7c33c5b653ef..bcb6279cbae7 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -609,13 +609,17 @@ static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
>> if (memcg == NULL) {
>> /*
>> * For non-memcg reclaim, is there
>> - * space in any swap device?
>> + * space in any swap device or swapcache pages?
>> */
>> - if (get_nr_swap_pages() > 0)
>> + if (get_nr_swap_pages() + total_swapcache_pages() > 0)
>> return true;
>> } else {
>> - /* Is the memcg below its swap limit? */
>> - if (mem_cgroup_get_nr_swap_pages(memcg) > 0)
>> + /*
>> + * Is the memcg below its swap limit or is there swapcache
>> + * pages can be freed?
>> + */
>> + if (mem_cgroup_get_nr_swap_pages(memcg) +
>> + mem_cgroup_get_nr_swapcache_pages(memcg) > 0)
>> return true;
>> }
>
> I wonder if it would be more efficient to set a bit in struct
> scan_control if we only are out of swap spaces but have swap cache
> pages, and only isolate anon pages that are in the swap cache, instead
> of isolating random anon pages. We may end up isolating pages that are
> not in the swap cache for a few iterations and wasting cycles.
Scanning swap cache directly will make the code more complex. IIUC, the
possibility for the swap device to be used up isn't high. If so, I
prefer the simpler implementation as that in this series.
--
Best Regards,
Huang, Ying
