2022-04-29 08:32:01

by Miaohe Lin

Subject: [PATCH 7/9] mm/z3fold: always clear PAGE_CLAIMED under z3fold page lock

Think about the below race window:

CPU1 CPU2
z3fold_reclaim_page z3fold_free
test_and_set_bit PAGE_CLAIMED
failed to reclaim page
z3fold_page_lock(zhdr);
add back to the lru list;
z3fold_page_unlock(zhdr);
get_z3fold_header
page_claimed=test_and_set_bit PAGE_CLAIMED

clear_bit(PAGE_CLAIMED, &page->private);

if (!page_claimed) /* it's false true */
free_handle is not called

free_handle won't be called in this case. So z3fold_buddy_slots will leak.
Fix it by always clear PAGE_CLAIMED under z3fold page lock.

Signed-off-by: Miaohe Lin <[email protected]>
---
mm/z3fold.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index a1c150fc8def..4a3cd2ff15b0 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -1221,8 +1221,8 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
return;
}
if (test_and_set_bit(NEEDS_COMPACTING, &page->private)) {
- put_z3fold_header(zhdr);
clear_bit(PAGE_CLAIMED, &page->private);
+ put_z3fold_header(zhdr);
return;
}
if (zhdr->cpu < 0 || !cpu_online(zhdr->cpu)) {
@@ -1424,8 +1424,8 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
spin_unlock(&pool->lock);
if (list_empty(&zhdr->buddy))
add_to_unbuddied(pool, zhdr);
- z3fold_page_unlock(zhdr);
clear_bit(PAGE_CLAIMED, &page->private);
+ z3fold_page_unlock(zhdr);
}

/* We started off locked to we need to lock the pool back */
@@ -1577,8 +1577,8 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
if (!z3fold_page_trylock(zhdr))
return -EAGAIN;
if (zhdr->mapped_count != 0 || zhdr->foreign_handles != 0) {
- z3fold_page_unlock(zhdr);
clear_bit(PAGE_CLAIMED, &page->private);
+ z3fold_page_unlock(zhdr);
return -EBUSY;
}
if (work_pending(&zhdr->work)) {
--
2.23.0

2022-05-19 13:47:05

by Vitaly Wool

Subject: Re: [PATCH 7/9] mm/z3fold: always clear PAGE_CLAIMED under z3fold page lock

On Fri, Apr 29, 2022 at 8:40 AM Miaohe Lin <[email protected]> wrote:
>
> Think about the below race window:
>
> CPU1 CPU2
> z3fold_reclaim_page z3fold_free
> test_and_set_bit PAGE_CLAIMED
> failed to reclaim page
> z3fold_page_lock(zhdr);
> add back to the lru list;
> z3fold_page_unlock(zhdr);
> get_z3fold_header
> page_claimed=test_and_set_bit PAGE_CLAIMED
>
> clear_bit(PAGE_CLAIMED, &page->private);
>
> if (!page_claimed) /* it's false true */
> free_handle is not called
>
> free_handle won't be called in this case. So z3fold_buddy_slots will leak.
> Fix it by always clear PAGE_CLAIMED under z3fold page lock.
>
> Signed-off-by: Miaohe Lin <[email protected]>
> ---
> mm/z3fold.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index a1c150fc8def..4a3cd2ff15b0 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -1221,8 +1221,8 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
> return;
> }
> if (test_and_set_bit(NEEDS_COMPACTING, &page->private)) {
> - put_z3fold_header(zhdr);
> clear_bit(PAGE_CLAIMED, &page->private);
> + put_z3fold_header(zhdr);
> return;
> }
> if (zhdr->cpu < 0 || !cpu_online(zhdr->cpu)) {
> @@ -1424,8 +1424,8 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
> spin_unlock(&pool->lock);
> if (list_empty(&zhdr->buddy))
> add_to_unbuddied(pool, zhdr);
> - z3fold_page_unlock(zhdr);
> clear_bit(PAGE_CLAIMED, &page->private);
> + z3fold_page_unlock(zhdr);
> }
>
> /* We started off locked to we need to lock the pool back */
> @@ -1577,8 +1577,8 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
> if (!z3fold_page_trylock(zhdr))
> return -EAGAIN;
> if (zhdr->mapped_count != 0 || zhdr->foreign_handles != 0) {
> - z3fold_page_unlock(zhdr);
> clear_bit(PAGE_CLAIMED, &page->private);
> + z3fold_page_unlock(zhdr);
> return -EBUSY;
> }
> if (work_pending(&zhdr->work)) {

Reviewed-by: Vitaly Wool <[email protected]>
> --
> 2.23.0
>