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SubjectRe: [PATCH v18 19/32] mm/swap.c: serialize memcg changes in pagevec_lru_move_fn
On Mon, 24 Aug 2020, Alex Shi 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(), that serializes

s/the ClearPageLRU/TestClearPageLRU/

> the memcg change. and then removing the PageLRU check in move_fn callee
> as the consequence.

Deserves another paragraph about __pagevec_lru_add():
"__pagevec_lru_add_fn() is different from the others, because the pages
it deals with are, by definition, not yet on the lru. TestClearPageLRU
is not needed and would not work, so __pagevec_lru_add() goes its own way."

>
> Reported-by: Hugh Dickins <hughd@google.com>

True.

> Signed-off-by: Hugh Dickins <hughd@google.com>

I did provide some lines, but I think it's just
Acked-by: Hugh Dickins <hughd@google.com>
to go below your Signed-off-by.

> 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 | 44 +++++++++++++++++++++++++++++++++++---------
> 1 file changed, 35 insertions(+), 9 deletions(-)

In your lruv19 branch, this patch got renamed (s/moveing/moving/):
but I think it's better with the old name used here in v18, and without
those mm/vmscan.c mods to check_move_unevictable_pages() tacked on:
please move those back to 16/32, which already makes changes to vmscan.c.

>
> diff --git a/mm/swap.c b/mm/swap.c
> index 446ffe280809..2d9a86bf93a4 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -221,8 +221,14 @@ static void pagevec_lru_move_fn(struct pagevec *pvec,
> spin_lock_irqsave(&pgdat->lru_lock, flags);
> }
>
> + /* block memcg migration during page moving between lru */
> + if (!TestClearPageLRU(page))
> + continue;
> +
> lruvec = mem_cgroup_page_lruvec(page, pgdat);
> (*move_fn)(page, lruvec);
> +
> + SetPageLRU(page);
> }
> if (pgdat)
> spin_unlock_irqrestore(&pgdat->lru_lock, flags);
> @@ -232,7 +238,7 @@ static void pagevec_lru_move_fn(struct pagevec *pvec,
>
> static void pagevec_move_tail_fn(struct page *page, struct lruvec *lruvec)
> {
> - if (PageLRU(page) && !PageUnevictable(page)) {
> + if (!PageUnevictable(page)) {
> del_page_from_lru_list(page, lruvec, page_lru(page));
> ClearPageActive(page);
> add_page_to_lru_list_tail(page, lruvec, page_lru(page));
> @@ -306,7 +312,7 @@ void lru_note_cost_page(struct page *page)
>
> static void __activate_page(struct page *page, struct lruvec *lruvec)
> {
> - if (PageLRU(page) && !PageActive(page) && !PageUnevictable(page)) {
> + if (!PageActive(page) && !PageUnevictable(page)) {
> int lru = page_lru_base_type(page);
> int nr_pages = thp_nr_pages(page);
>
> @@ -362,7 +368,8 @@ void activate_page(struct page *page)
>
> page = compound_head(page);
> spin_lock_irq(&pgdat->lru_lock);
> - __activate_page(page, mem_cgroup_page_lruvec(page, pgdat));
> + if (PageLRU(page))
> + __activate_page(page, mem_cgroup_page_lruvec(page, pgdat));
> spin_unlock_irq(&pgdat->lru_lock);
> }
> #endif

Every time I look at this, I wonder if that's right, or an unnecessary
optimization strayed in, or whatever. For the benefit of others looking
at this patch, yes it is right: this is the !CONFIG_SMP alternative
version of activate_page(), and needs that PageLRU check to compensate
for the check that has now been removed from __activate_page() itself.

> @@ -521,9 +528,6 @@ static void lru_deactivate_file_fn(struct page *page, struct lruvec *lruvec)
> bool active;
> int nr_pages = thp_nr_pages(page);
>
> - if (!PageLRU(page))
> - return;
> -
> if (PageUnevictable(page))
> return;
>
> @@ -564,7 +568,7 @@ static void lru_deactivate_file_fn(struct page *page, struct lruvec *lruvec)
>
> static void lru_deactivate_fn(struct page *page, struct lruvec *lruvec)
> {
> - if (PageLRU(page) && PageActive(page) && !PageUnevictable(page)) {
> + if (PageActive(page) && !PageUnevictable(page)) {
> int lru = page_lru_base_type(page);
> int nr_pages = thp_nr_pages(page);
>
> @@ -581,7 +585,7 @@ static void lru_deactivate_fn(struct page *page, struct lruvec *lruvec)
>
> static void lru_lazyfree_fn(struct page *page, struct lruvec *lruvec)
> {
> - if (PageLRU(page) && PageAnon(page) && PageSwapBacked(page) &&
> + if (PageAnon(page) && PageSwapBacked(page) &&
> !PageSwapCache(page) && !PageUnevictable(page)) {
> bool active = PageActive(page);
> int nr_pages = thp_nr_pages(page);
> @@ -979,7 +983,29 @@ 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);
> + int i;
> + struct pglist_data *pgdat = NULL;
> + struct lruvec *lruvec;
> + unsigned long flags = 0;
> +
> + for (i = 0; i < pagevec_count(pvec); i++) {
> + struct page *page = pvec->pages[i];
> + struct pglist_data *pagepgdat = page_pgdat(page);
> +
> + if (pagepgdat != pgdat) {
> + if (pgdat)
> + spin_unlock_irqrestore(&pgdat->lru_lock, flags);
> + pgdat = pagepgdat;
> + spin_lock_irqsave(&pgdat->lru_lock, flags);
> + }
> +
> + lruvec = mem_cgroup_page_lruvec(page, pgdat);
> + __pagevec_lru_add_fn(page, lruvec);
> + }
> + if (pgdat)
> + spin_unlock_irqrestore(&pgdat->lru_lock, flags);
> + release_pages(pvec->pages, pvec->nr);
> + pagevec_reinit(pvec);
> }
>
> /**
> --
> 1.8.3.1

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