Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:
> On Thu, Aug 24, 2023 at 1:51 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> 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.
>
> I did not mean that, sorry if I wasn't clear. I meant to set a bit in
> struct scan_control, and then in isolate_lru_folios() for anon lrus,
> we can skip isolating folios that are not in the swapcache if that bit
> is set.
>
> My main concern was that if we have a few pages in the swapcache we
> may end up wasting cycles scanning through a lot of other anonymous
> pages until we reach them. If that's too much complexity that's
> understandable.
Sorry, I misunderstood your idea. This sounds reasonable to me. We can
check swap space and swap cache in isolate_lru_folios(), either in
isolate_lru_folios() directly, or via a bit in scan_control.
--
Best Regards,
Huang, Ying
