Re: [PATCH v14 15/20] mm/swap: serialize memcg changes during pagevec_lru_move_fn

Konstantin Khlebnikov
2020-07-03T09:13:27Z

On Fri, Jul 3, 2020 at 8:09 AM Alex Shi <alex.shi@linux.alibaba.com> wrote:
>
> Hugh Dickins' found a memcg change bug on original version:
> If we want to change the pgdat->lru_lock to memcg's lruvec lock, we have
> to serialize mem_cgroup_move_account during pagevec_lru_move_fn. The
> possible bad scenario would like:
>
> cpu 0 cpu 1
> lruvec = mem_cgroup_page_lruvec()
> if (!isolate_lru_page())
> mem_cgroup_move_account
>
> spin_lock_irqsave(&lruvec->lru_lock <== wrong lock.
>
> So we need the ClearPageLRU to block isolate_lru_page(), then serialize
> the memcg change here.
>
> Reported-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
> mm/swap.c | 31 +++++++++++++++++++------------
> 1 file changed, 19 insertions(+), 12 deletions(-)
>
> diff --git a/mm/swap.c b/mm/swap.c
> index b24d5f69b93a..55eb2c2eed03 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -203,7 +203,7 @@ int get_kernel_page(unsigned long start, int write, struct page **pages)
> EXPORT_SYMBOL_GPL(get_kernel_page);
>
> static void pagevec_lru_move_fn(struct pagevec *pvec,
> - void (*move_fn)(struct page *page, struct lruvec *lruvec))
> + void (*move_fn)(struct page *page, struct lruvec *lruvec), bool add)
> {
> int i;
> struct pglist_data *pgdat = NULL;
> @@ -221,8 +221,15 @@ static void pagevec_lru_move_fn(struct pagevec *pvec,
> spin_lock_irqsave(&pgdat->lru_lock, flags);
> }
>
> + /* new page add to lru or page moving between lru */
> + if (!add && !TestClearPageLRU(page))
> + continue;
> +
> lruvec = mem_cgroup_page_lruvec(page, pgdat);
> (*move_fn)(page, lruvec);
> +
> + if (!add)
> + SetPageLRU(page);
> }
> if (pgdat)
> spin_unlock_irqrestore(&pgdat->lru_lock, flags);
> @@ -259,7 +266,7 @@ void rotate_reclaimable_page(struct page *page)
> local_lock_irqsave(&lru_rotate.lock, flags);
> pvec = this_cpu_ptr(&lru_rotate.pvec);
> if (!pagevec_add(pvec, page) || PageCompound(page))
> - pagevec_lru_move_fn(pvec, pagevec_move_tail_fn);
> + pagevec_lru_move_fn(pvec, pagevec_move_tail_fn, false);
> local_unlock_irqrestore(&lru_rotate.lock, flags);
> }
> }
> @@ -328,7 +335,7 @@ static void activate_page_drain(int cpu)
> struct pagevec *pvec = &per_cpu(lru_pvecs.activate_page, cpu);
>
> if (pagevec_count(pvec))
> - pagevec_lru_move_fn(pvec, __activate_page);
> + pagevec_lru_move_fn(pvec, __activate_page, false);
> }
>
> static bool need_activate_page_drain(int cpu)
> @@ -346,7 +353,7 @@ void activate_page(struct page *page)
> pvec = this_cpu_ptr(&lru_pvecs.activate_page);
> get_page(page);
> if (!pagevec_add(pvec, page) || PageCompound(page))
> - pagevec_lru_move_fn(pvec, __activate_page);
> + pagevec_lru_move_fn(pvec, __activate_page, false);
> local_unlock(&lru_pvecs.lock);
> }
> }
> @@ -621,21 +628,21 @@ void lru_add_drain_cpu(int cpu)
>
> /* No harm done if a racing interrupt already did this */
> local_lock_irqsave(&lru_rotate.lock, flags);
> - pagevec_lru_move_fn(pvec, pagevec_move_tail_fn);
> + pagevec_lru_move_fn(pvec, pagevec_move_tail_fn, false);
> local_unlock_irqrestore(&lru_rotate.lock, flags);
> }
>
> pvec = &per_cpu(lru_pvecs.lru_deactivate_file, cpu);
> if (pagevec_count(pvec))
> - pagevec_lru_move_fn(pvec, lru_deactivate_file_fn);
> + pagevec_lru_move_fn(pvec, lru_deactivate_file_fn, false);
>
> pvec = &per_cpu(lru_pvecs.lru_deactivate, cpu);
> if (pagevec_count(pvec))
> - pagevec_lru_move_fn(pvec, lru_deactivate_fn);
> + pagevec_lru_move_fn(pvec, lru_deactivate_fn, false);
>
> pvec = &per_cpu(lru_pvecs.lru_lazyfree, cpu);
> if (pagevec_count(pvec))
> - pagevec_lru_move_fn(pvec, lru_lazyfree_fn);
> + pagevec_lru_move_fn(pvec, lru_lazyfree_fn, false);
>
> activate_page_drain(cpu);
> }
> @@ -664,7 +671,7 @@ void deactivate_file_page(struct page *page)
> pvec = this_cpu_ptr(&lru_pvecs.lru_deactivate_file);
>
> if (!pagevec_add(pvec, page) || PageCompound(page))
> - pagevec_lru_move_fn(pvec, lru_deactivate_file_fn);
> + pagevec_lru_move_fn(pvec, lru_deactivate_file_fn, false);
> local_unlock(&lru_pvecs.lock);
> }
> }
> @@ -686,7 +693,7 @@ void deactivate_page(struct page *page)
> pvec = this_cpu_ptr(&lru_pvecs.lru_deactivate);
> get_page(page);
> if (!pagevec_add(pvec, page) || PageCompound(page))
> - pagevec_lru_move_fn(pvec, lru_deactivate_fn);
> + pagevec_lru_move_fn(pvec, lru_deactivate_fn, false);
> local_unlock(&lru_pvecs.lock);
> }
> }
> @@ -708,7 +715,7 @@ void mark_page_lazyfree(struct page *page)
> pvec = this_cpu_ptr(&lru_pvecs.lru_lazyfree);
> get_page(page);
> if (!pagevec_add(pvec, page) || PageCompound(page))
> - pagevec_lru_move_fn(pvec, lru_lazyfree_fn);
> + pagevec_lru_move_fn(pvec, lru_lazyfree_fn, false);
> local_unlock(&lru_pvecs.lock);
> }
> }
> @@ -976,7 +983,7 @@ static void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec)
> */
> void __pagevec_lru_add(struct pagevec *pvec)
> {
> - pagevec_lru_move_fn(pvec, __pagevec_lru_add_fn);
> + pagevec_lru_move_fn(pvec, __pagevec_lru_add_fn, true);
> }

It seems better to open code version in lru_add than adding a bool
argument which is true just for one user.

Also with this new lru protection logic lru_add could be optimized:
It could prepare a list of pages and under lru_lock do only list
splice and bumping counter.
Since PageLRU isn't set yet nobody could touch these pages in lru.
After that lru_add could iterate pages from first to last without
lru_lock to set PageLRU and drop reference.

So, lru_add will do O(1) operations under lru_lock regardless of the
count of pages it added.

Actually per-cpu vector for adding could be replaced with per-cpu
lists and\or per-lruvec atomic slist.
Thus incommig pages will be already in list structure rather than page vector.
This allows to accumulate more pages and offload adding to kswapd or
direct reclaim.

>
> /**
> --
> 1.8.3.1
>
>