Re: [PATCH v2] Revert "mm: skip CMA pages when they are not available"

[Barry Song <21cnbao@gmail.com>]

On Fri, Mar 15, 2024 at 9:18 PM <liuhailong@oppo.com> wrote:
>
> From: "Hailong.Liu" <liuhailong@oppo.com>
>
> This reverts
> commit b7108d66318a ("Multi-gen LRU: skip CMA pages when they are not eligible")
> commit 5da226dbfce3 ("mm: skip CMA pages when they are not available")

Reverting these changes seems inappropriate since the original patches
did improve most cases.
While doing direct reclamation without MOVABLE flags, scanning cma
folio doesn't help at all.

>
> skip_cma may cause system not responding. if cma pages is large in lru_list
> and system is in lowmemory, many tasks would direct reclaim and waste
> cpu time to isolate_lru_pages and return.
>
> Test this patch on android-5.15 8G device
> reproducer:
> - cma_declare_contiguous 3G pages
> - set /proc/sys/vm/swappiness 0 to enable direct_reclaim reclaim file
>   only.
> - run a memleak process in userspace


I assume that scanning cma folio provides additional opportunities to reclaim
anonymous memory, even though it is ineffective for allocating NON-MOVABLE
folios. Consequently, we alleviate pressure for future allocations of anonymous
folios. Moreover, setting swappiness=0 is a rare scenario. Instead of entirely
removing the mechanism, could we detect such corner cases and handle them
differently.

Otherwise, I don't see any chance of this being acknowledged.

>
> trace the trace_mm_vmscan_lru_isolate which get
> [ 1178.059160] 2825  3405 I UsbFfs-worker   : isolate_lru_pages:1895 total_scan: 24385 skipped: 24382
> [ 1178.059699] 2825  3405 I UsbFfs-worker   : isolate_lru_pages:1895 total_scan: 24412 skipped: 24401
> [ 1178.061747] 2825  3405 I UsbFfs-worker   : isolate_lru_pages:1895 total_scan: 24412 skipped: 24401
> From the log, tasks is waste time to scan lru list and call stack as below.
>
> Task name: UsbFfs-worker [affinity: 0xff] pid: 3374 cpu: 7 prio: 120 start: ffffff8897a35c80
> state: 0x0[R] exit_state: 0x0 stack base: 0xffffffc01eaa0000
> Last_enqueued_ts:       0.000000000 Last_sleep_ts:       0.000000000
> Stack:
> [<ffffffd32ee7d910>] __switch_to+0x180
> [<ffffffd3302022fc>] __schedule+0x4dc
> [<ffffffd330201e08>] preempt_schedule+0x5c
> [<ffffffd33020a4d0>] _raw_spin_unlock_irq+0x54
> [<ffffffd32f14906c>] shrink_inactive_list+0x1d0
> [<ffffffd32f143998>] shrink_lruvec+0x1bc
> [<ffffffd32f147c0c>] shrink_node_memcgs+0x184
> [<ffffffd32f147414>] shrink_node+0x2d0
> [<ffffffd32f146d38>] shrink_zones+0x14c
> [<ffffffd32f142e84>] do_try_to_free_pages+0xe8
> [<ffffffd32f142b08>] try_to_free_pages+0x2e0
> [<ffffffd32f1a8e44>] __alloc_pages_direct_reclaim+0x84
> [<ffffffd32f1a2d58>] __alloc_pages_slowpath+0x4d0
> [<ffffffd32f1a23bc>] __alloc_pages_nodemask[jt]+0x124
> [<ffffffd32f19a220>] __vmalloc_area_node+0x188
> [<ffffffd32f19a540>] __vmalloc_node+0x148
> [<ffffffd32f19a60c>] vmalloc+0x4c
> [<ffffffd32f910218>] ffs_epfile_io+0x258
> [<ffffffd330033780>] kretprobe_trampoline[jt]+0x0
> [<ffffffd330033780>] kretprobe_trampoline[jt]+0x0
> [<ffffffd32f28129c>] __io_submit_one+0x1c0
> [<ffffffd32f280e38>] io_submit_one+0x88
> [<ffffffd32f280c88>] __do_sys_io_submit+0x178
> [<ffffffd32f27eac0>] __arm64_sys_io_submit+0x20
> [<ffffffd32eeabb74>] el0_svc_common.llvm.9961749221945255377+0xd0
> [<ffffffd32eeaba34>] do_el0_svc+0x28
> [<ffffffd32ff21be8>] el0_svc+0x14
> [<ffffffd32ff21b70>] el0_sync_handler+0x88
> [<ffffffd32ee128b8>] el0_sync+0x1b8
>
> Task name: kthreadd [affinity: 0xff] pid: 2 cpu: 7 prio: 120 start: ffffff87808c0000
> state: 0x0[R] exit_state: 0x0 stack base: 0xffffffc008078000
> Last_enqueued_ts:       0.000000000 Last_sleep_ts:       0.000000000
> Stack:
> [<ffffffd32ee7d910>] __switch_to+0x180
> [<ffffffd3302022fc>] __schedule+0x4dc
> [<ffffffd330201e08>] preempt_schedule+0x5c
> [<ffffffd33020a4d0>] _raw_spin_unlock_irq+0x54
> [<ffffffd32f149168>] shrink_inactive_list+0x2cc
> [<ffffffd32f143998>] shrink_lruvec+0x1bc
> [<ffffffd32f147c0c>] shrink_node_memcgs+0x184
> [<ffffffd32f147414>] shrink_node+0x2d0
> [<ffffffd32f146d38>] shrink_zones+0x14c
> [<ffffffd32f142e84>] do_try_to_free_pages+0xe8
> [<ffffffd32f142b08>] try_to_free_pages+0x2e0
> [<ffffffd32f1a8e44>] __alloc_pages_direct_reclaim+0x84
> [<ffffffd32f1a2d58>] __alloc_pages_slowpath+0x4d0
> [<ffffffd32f1a23bc>] __alloc_pages_nodemask[jt]+0x124
> [<ffffffd32f19a220>] __vmalloc_area_node+0x188
> [<ffffffd32f19a044>] __vmalloc_node_range+0x88
> [<ffffffd32f0fb430>] scs_alloc+0x1b8
> [<ffffffd32f0fb62c>] scs_prepare+0x20
> [<ffffffd32ef2ce04>] dup_task_struct+0xd4
> [<ffffffd32ef2a77c>] copy_process+0x144
> [<ffffffd32ef2bae4>] kernel_clone+0xb4
> [<ffffffd32ef2c040>] kernel_thread+0x5c
> [<ffffffd32ef618d0>] kthreadd+0x184
>
> Signed-off-by: Hailong.Liu <liuhailong@oppo.com>
> ---
>  mm/vmscan.c | 24 ++----------------------
>  1 file changed, 2 insertions(+), 22 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 6f13394b112e..29306c29309f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2261,25 +2261,6 @@ static __always_inline void update_lru_sizes(struct lruvec *lruvec,
>
>  }
>
> -#ifdef CONFIG_CMA
> -/*
> - * It is waste of effort to scan and reclaim CMA pages if it is not available
> - * for current allocation context. Kswapd can not be enrolled as it can not
> - * distinguish this scenario by using sc->gfp_mask = GFP_KERNEL
> - */
> -static bool skip_cma(struct folio *folio, struct scan_control *sc)
> -{
> -       return !current_is_kswapd() &&
> -                       gfp_migratetype(sc->gfp_mask) != MIGRATE_MOVABLE &&
> -                       get_pageblock_migratetype(&folio->page) == MIGRATE_CMA;
> -}
> -#else
> -static bool skip_cma(struct folio *folio, struct scan_control *sc)
> -{
> -       return false;
> -}
> -#endif
> -
>  /*
>   * Isolating page from the lruvec to fill in @dst list by nr_to_scan times.
>   *
> @@ -2326,8 +2307,7 @@ static unsigned long isolate_lru_folios(unsigned long nr_to_scan,
>                 nr_pages = folio_nr_pages(folio);
>                 total_scan += nr_pages;
>
> -               if (folio_zonenum(folio) > sc->reclaim_idx ||
> -                               skip_cma(folio, sc)) {
> +               if (folio_zonenum(folio) > sc->reclaim_idx) {
>                         nr_skipped[folio_zonenum(folio)] += nr_pages;
>                         move_to = &folios_skipped;
>                         goto move;
> @@ -4945,7 +4925,7 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
>         }
>
>         /* ineligible */
> -       if (zone > sc->reclaim_idx || skip_cma(folio, sc)) {
> +       if (zone > sc->reclaim_idx) {
>                 gen = folio_inc_gen(lruvec, folio, false);
>                 list_move_tail(&folio->lru, &lrugen->folios[gen][type][zone]);
>                 return true;
> --
> Changes in v2:
> - add reproducer
> - fix build error on v6.6-rc7
>
>

Thanks
Barry