[PATCH -V3 -mm] mm, swap: Fix race between swapoff and some swap operations

From: Huang, Ying
Date: Mon Dec 18 2017 - 02:34:44 EST

From: Huang Ying <ying.huang@xxxxxxxxx>

When the swapin is performed, after getting the swap entry information
from the page table, system will swap in the swap entry, without any
lock held to prevent the swap device from being swapoff. This may
cause the race like below,

CPU 1 CPU 2
----- -----
do_swap_page
swapin_readahead
__read_swap_cache_async
swapoff swapcache_prepare
p->swap_map = NULL __swap_duplicate
p->swap_map[?] /* !!! NULL pointer access */

Because swapoff is usually done when system shutdown only, the race
may not hit many people in practice. But it is still a race need to
be fixed.

To fix the race, get_swap_device() is added to check whether the
specified swap entry is valid in its swap device. If so, it will keep
the swap entry valid via preventing the swap device from being
swapoff, until put_swap_device() is called.

Because swapoff() is very race code path, to make the normal path runs
as fast as possible, RCU instead of reference count is used to
implement get/put_swap_device(). From get_swap_device() to
put_swap_device(), the RCU read lock is held, so synchronize_rcu() in
swapoff() will wait until put_swap_device() is called.

In addition to swap_map, cluster_info, etc. data structure in the
struct swap_info_struct, the swap cache radix tree will be freed after
swapoff, so this patch fixes the race between swap cache looking up
and swapoff too.

Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Shaohua Li <shli@xxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: "JÃrÃme Glisse" <jglisse@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
Cc: Aaron Lu <aaron.lu@xxxxxxxxx>
Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

Changelog:

v3:

- Re-implemented with RCU to reduce the overhead of normal paths

v2:

- Re-implemented with SRCU to reduce the overhead of normal paths.

- Avoid to check whether the swap device has been swapoff in
get_swap_device(). Because we can check the origin of the swap
entry to make sure the swap device hasn't bee swapoff.
---
include/linux/swap.h | 11 +++++-
mm/memory.c | 2 +-
mm/swap_state.c | 16 ++++++--
mm/swapfile.c | 105 +++++++++++++++++++++++++++++++++++++++------------
4 files changed, 103 insertions(+), 31 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 2417d288e016..f7e8f26cf07f 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -172,8 +172,9 @@ enum {
SWP_PAGE_DISCARD = (1 << 9), /* freed swap page-cluster discards */
SWP_STABLE_WRITES = (1 << 10), /* no overwrite PG_writeback pages */
SWP_SYNCHRONOUS_IO = (1 << 11), /* synchronous IO is efficient */
+ SWP_VALID = (1 << 12), /* swap is valid to be operated on? */
/* add others here before... */
- SWP_SCANNING = (1 << 12), /* refcount in scan_swap_map */
+ SWP_SCANNING = (1 << 13), /* refcount in scan_swap_map */
};

#define SWAP_CLUSTER_MAX 32UL
@@ -460,7 +461,7 @@ extern unsigned int count_swap_pages(int, int);
extern sector_t map_swap_page(struct page *, struct block_device **);
extern sector_t swapdev_block(int, pgoff_t);
extern int page_swapcount(struct page *);
-extern int __swap_count(struct swap_info_struct *si, swp_entry_t entry);
+extern int __swap_count(swp_entry_t entry);
extern int __swp_swapcount(swp_entry_t entry);
extern int swp_swapcount(swp_entry_t entry);
extern struct swap_info_struct *page_swap_info(struct page *);
@@ -470,6 +471,12 @@ extern int try_to_free_swap(struct page *);
struct backing_dev_info;
extern int init_swap_address_space(unsigned int type, unsigned long nr_pages);
extern void exit_swap_address_space(unsigned int type);
+extern struct swap_info_struct *get_swap_device(swp_entry_t entry);
+
+static inline void put_swap_device(struct swap_info_struct *si)
+{
+ rcu_read_unlock();
+}

#else /* CONFIG_SWAP */

diff --git a/mm/memory.c b/mm/memory.c
index 1a969992f76b..77a7d6191218 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2909,7 +2909,7 @@ int do_swap_page(struct vm_fault *vmf)
struct swap_info_struct *si = swp_swap_info(entry);

if (si->flags & SWP_SYNCHRONOUS_IO &&
- __swap_count(si, entry) == 1) {
+ __swap_count(entry) == 1) {
/* skip swapcache */
page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma,
vmf->address);
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 0b8ae361981f..8dde719e973c 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -337,8 +337,13 @@ struct page *lookup_swap_cache(swp_entry_t entry, struct vm_area_struct *vma,
unsigned long addr)
{
struct page *page;
+ struct swap_info_struct *si;

+ si = get_swap_device(entry);
+ if (!si)
+ return NULL;
page = find_get_page(swap_address_space(entry), swp_offset(entry));
+ put_swap_device(si);

INC_CACHE_INFO(find_total);
if (page) {
@@ -376,8 +381,8 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
struct vm_area_struct *vma, unsigned long addr,
bool *new_page_allocated)
{
- struct page *found_page, *new_page = NULL;
- struct address_space *swapper_space = swap_address_space(entry);
+ struct page *found_page = NULL, *new_page = NULL;
+ struct swap_info_struct *si;
int err;
*new_page_allocated = false;

@@ -387,7 +392,12 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
* called after lookup_swap_cache() failed, re-calling
* that would confuse statistics.
*/
- found_page = find_get_page(swapper_space, swp_offset(entry));
+ si = get_swap_device(entry);
+ if (!si)
+ break;
+ found_page = find_get_page(swap_address_space(entry),
+ swp_offset(entry));
+ put_swap_device(si);
if (found_page)
break;

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 42fe5653814a..ca7b4c5ebe34 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1107,6 +1107,46 @@ static struct swap_info_struct *swap_info_get_cont(swp_entry_t entry,
return p;
}

+/*
+ * Check whether swap entry is valid in the swap device. If so,
+ * return pointer to swap_info_struct, and keep the swap entry valid
+ * via preventing the swap device from being swapoff, until
+ * put_swap_device() is called. Otherwise return NULL.
+ */
+struct swap_info_struct *get_swap_device(swp_entry_t entry)
+{
+ struct swap_info_struct *si;
+ unsigned long type, offset;
+
+ if (!entry.val)
+ goto out;
+ type = swp_type(entry);
+ if (type >= nr_swapfiles)
+ goto bad_nofile;
+ si = swap_info[type];
+
+ rcu_read_lock();
+ if (!(si->flags & SWP_VALID))
+ goto unlock_out;
+ /*
+ * Corresponds smp_wmb() in _enable_swap_info() to make sure
+ * swap_map, cluster_info, etc. are read after flags is read
+ */
+ smp_rmb();
+ offset = swp_offset(entry);
+ if (offset >= si->max)
+ goto unlock_out;
+
+ return si;
+bad_nofile:
+ pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
+out:
+ return NULL;
+unlock_out:
+ rcu_read_unlock();
+ return NULL;
+}
+
static unsigned char __swap_entry_free(struct swap_info_struct *p,
swp_entry_t entry, unsigned char usage)
{
@@ -1328,11 +1368,18 @@ int page_swapcount(struct page *page)
return count;
}

-int __swap_count(struct swap_info_struct *si, swp_entry_t entry)
+int __swap_count(swp_entry_t entry)
{
+ struct swap_info_struct *si;
pgoff_t offset = swp_offset(entry);
+ int count = 0;

- return swap_count(si->swap_map[offset]);
+ si = get_swap_device(entry);
+ if (si) {
+ count = swap_count(si->swap_map[offset]);
+ put_swap_device(si);
+ }
+ return count;
}

static int swap_swapcount(struct swap_info_struct *si, swp_entry_t entry)
@@ -1357,9 +1404,11 @@ int __swp_swapcount(swp_entry_t entry)
int count = 0;
struct swap_info_struct *si;

- si = __swap_info_get(entry);
- if (si)
+ si = get_swap_device(entry);
+ if (si) {
count = swap_swapcount(si, entry);
+ put_swap_device(si);
+ }
return count;
}

@@ -2478,7 +2527,9 @@ static void _enable_swap_info(struct swap_info_struct *p, int prio,
}
p->swap_map = swap_map;
p->cluster_info = cluster_info;
- p->flags |= SWP_WRITEOK;
+ /* Correspond to smp_rmb() in get_swap_device(), check it for details */
+ smp_wmb();
+ p->flags |= SWP_WRITEOK | SWP_VALID;
atomic_long_add(p->pages, &nr_swap_pages);
total_swap_pages += p->pages;

@@ -2617,6 +2668,17 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)

reenable_swap_slots_cache_unlock();

+ spin_lock(&swap_lock);
+ spin_lock(&p->lock);
+ p->flags &= ~SWP_VALID; /* mark swap device as invalid */
+ spin_unlock(&p->lock);
+ spin_unlock(&swap_lock);
+ /*
+ * wait for swap operations protected by get/put_swap_device()
+ * to complete
+ */
+ synchronize_rcu();
+
flush_work(&p->discard_work);

destroy_swap_extents(p);
@@ -3356,22 +3418,16 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
{
struct swap_info_struct *p;
struct swap_cluster_info *ci;
- unsigned long offset, type;
+ unsigned long offset;
unsigned char count;
unsigned char has_cache;
int err = -EINVAL;

- if (non_swap_entry(entry))
+ p = get_swap_device(entry);
+ if (!p)
goto out;

- type = swp_type(entry);
- if (type >= nr_swapfiles)
- goto bad_file;
- p = swap_info[type];
offset = swp_offset(entry);
- if (unlikely(offset >= p->max))
- goto out;
-
ci = lock_cluster_or_swap_info(p, offset);

count = p->swap_map[offset];
@@ -3417,11 +3473,9 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
unlock_out:
unlock_cluster_or_swap_info(p, ci);
out:
+ if (p)
+ put_swap_device(p);
return err;
-
-bad_file:
- pr_err("swap_dup: %s%08lx\n", Bad_file, entry.val);
- goto out;
}

/*
@@ -3513,6 +3567,7 @@ int add_swap_count_continuation(swp_entry_t entry, gfp_t gfp_mask)
struct page *list_page;
pgoff_t offset;
unsigned char count;
+ int ret = 0;

/*
* When debugging, it's easier to use __GFP_ZERO here; but it's better
@@ -3520,15 +3575,15 @@ int add_swap_count_continuation(swp_entry_t entry, gfp_t gfp_mask)
*/
page = alloc_page(gfp_mask | __GFP_HIGHMEM);

- si = swap_info_get(entry);
+ si = get_swap_device(entry);
if (!si) {
/*
* An acceptable race has occurred since the failing
- * __swap_duplicate(): the swap entry has been freed,
- * perhaps even the whole swap_map cleared for swapoff.
+ * __swap_duplicate(): the swap device may be swapoff
*/
goto outer;
}
+ spin_lock(&si->lock);

offset = swp_offset(entry);

@@ -3546,9 +3601,8 @@ int add_swap_count_continuation(swp_entry_t entry, gfp_t gfp_mask)
}

if (!page) {
- unlock_cluster(ci);
- spin_unlock(&si->lock);
- return -ENOMEM;
+ ret = -ENOMEM;
+ goto out;
}

/*
@@ -3600,10 +3654,11 @@ int add_swap_count_continuation(swp_entry_t entry, gfp_t gfp_mask)
out:
unlock_cluster(ci);
spin_unlock(&si->lock);
+ put_swap_device(si);
outer:
if (page)
__free_page(page);
- return 0;
+ return ret;
}

/*
--
2.15.0