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SubjectRe: [PATCH -mm -V7] mm, swap: fix race between swapoff and some swap operations
On Mon 11-02-19 16:38:46, Huang, Ying wrote:
> From: Huang Ying <ying.huang@intel.com>
>
> When 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 rare code path, to make the normal path runs as
> fast as possible, disabling preemption + stop_machine() instead of
> reference count is used to implement get/put_swap_device(). From
> get_swap_device() to put_swap_device(), the preemption is disabled, so
> stop_machine() 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.
>
> Races between some other swap cache usages protected via disabling
> preemption and swapoff are fixed too via calling stop_machine() between
> clearing PageSwapCache() and freeing swap cache data structure.
>
> Alternative implementation could be replacing disable preemption with
> rcu_read_lock_sched and stop_machine() with synchronize_sched().

using stop_machine is generally discouraged. It is a gross
synchronization.

Besides that, since when do we have this problem?

> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Not-Nacked-by: Hugh Dickins <hughd@google.com>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Jérôme Glisse <jglisse@redhat.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Dave Jiang <dave.jiang@intel.com>
> Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
> Cc: Andrea Parri <andrea.parri@amarulasolutions.com>
>
> Changelog:
>
> v7:
>
> - Rebased on patch: "mm, swap: bounds check swap_info accesses to avoid NULL derefs"
>
> v6:
>
> - Add more comments to get_swap_device() to make it more clear about
> possible swapoff or swapoff+swapon.
>
> v5:
>
> - Replace RCU with stop_machine()
>
> v4:
>
> - Use synchronize_rcu() in enable_swap_info() to reduce overhead of
> normal paths further.
>
> 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 | 13 +++-
> mm/memory.c | 2 +-
> mm/swap_state.c | 16 ++++-
> mm/swapfile.c | 150 ++++++++++++++++++++++++++++++++++---------
> 4 files changed, 145 insertions(+), 36 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 649529be91f2..aecd1430d0a9 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -175,8 +175,9 @@ enum {
> SWP_PAGE_DISCARD = (1 << 10), /* freed swap page-cluster discards */
> SWP_STABLE_WRITES = (1 << 11), /* no overwrite PG_writeback pages */
> SWP_SYNCHRONOUS_IO = (1 << 12), /* synchronous IO is efficient */
> + SWP_VALID = (1 << 13), /* swap is valid to be operated on? */
> /* add others here before... */
> - SWP_SCANNING = (1 << 13), /* refcount in scan_swap_map */
> + SWP_SCANNING = (1 << 14), /* 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)
> +{
> + preempt_enable();
> +}
>
> #else /* CONFIG_SWAP */
>
> @@ -576,7 +583,7 @@ static inline int page_swapcount(struct page *page)
> return 0;
> }
>
> -static inline int __swap_count(struct swap_info_struct *si, swp_entry_t entry)
> +static inline int __swap_count(swp_entry_t entry)
> {
> return 0;
> }
> diff --git a/mm/memory.c b/mm/memory.c
> index 34ced1369883..9c0743c17c6c 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2719,7 +2719,7 @@ vm_fault_t 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 85245fdec8d9..61453f1faf72 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -310,8 +310,13 @@ 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) {
> @@ -354,8 +359,8 @@ 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;
>
> @@ -365,7 +370,12 @@ 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 cca8420b12db..8f92f36814fb 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -38,6 +38,7 @@
> #include <linux/export.h>
> #include <linux/swap_slots.h>
> #include <linux/sort.h>
> +#include <linux/stop_machine.h>
>
> #include <asm/pgtable.h>
> #include <asm/tlbflush.h>
> @@ -1186,6 +1187,64 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
> return usage;
> }
>
> +/*
> + * 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.
> + *
> + * Notice that swapoff or swapoff+swapon can still happen before the
> + * preempt_disable() in get_swap_device() or after the
> + * preempt_enable() in put_swap_device() if there isn't any other way
> + * to prevent swapoff, such as page lock, page table lock, etc. The
> + * caller must be prepared for that. For example, the following
> + * situation is possible.
> + *
> + * CPU1 CPU2
> + * do_swap_page()
> + * ... swapoff+swapon
> + * __read_swap_cache_async()
> + * swapcache_prepare()
> + * __swap_duplicate()
> + * // check swap_map
> + * // verify PTE not changed
> + *
> + * In __swap_duplicate(), the swap_map need to be checked before
> + * changing partly because the specified swap entry may be for another
> + * swap device which has been swapoff. And in do_swap_page(), after
> + * the page is read from the swap device, the PTE is verified not
> + * changed with the page table locked to check whether the swap device
> + * has been swapoff or swapoff+swapon.
> + */
> +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);
> + si = swap_type_to_swap_info(type);
> + if (!si)
> + goto bad_nofile;
> +
> + preempt_disable();
> + if (!(si->flags & SWP_VALID))
> + goto unlock_out;
> + 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:
> + preempt_enable();
> + return NULL;
> +}
> +
> static unsigned char __swap_entry_free(struct swap_info_struct *p,
> swp_entry_t entry, unsigned char usage)
> {
> @@ -1357,11 +1416,18 @@ 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)
> @@ -1386,9 +1452,11 @@ 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;
> }
>
> @@ -2332,9 +2400,9 @@ static int swap_node(struct swap_info_struct *p)
> return bdev ? bdev->bd_disk->node_id : NUMA_NO_NODE;
> }
>
> -static void _enable_swap_info(struct swap_info_struct *p, int prio,
> - unsigned char *swap_map,
> - struct swap_cluster_info *cluster_info)
> +static void setup_swap_info(struct swap_info_struct *p, int prio,
> + unsigned char *swap_map,
> + struct swap_cluster_info *cluster_info)
> {
> int i;
>
> @@ -2359,7 +2427,11 @@ 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;
> +}
> +
> +static void _enable_swap_info(struct swap_info_struct *p)
> +{
> + p->flags |= SWP_WRITEOK | SWP_VALID;
> atomic_long_add(p->pages, &nr_swap_pages);
> total_swap_pages += p->pages;
>
> @@ -2378,6 +2450,11 @@ static void _enable_swap_info(struct swap_info_struct *p, int prio,
> add_to_avail_list(p);
> }
>
> +static int swap_onoff_stop(void *arg)
> +{
> + return 0;
> +}
> +
> static void enable_swap_info(struct swap_info_struct *p, int prio,
> unsigned char *swap_map,
> struct swap_cluster_info *cluster_info,
> @@ -2386,7 +2463,17 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
> frontswap_init(p->type, frontswap_map);
> spin_lock(&swap_lock);
> spin_lock(&p->lock);
> - _enable_swap_info(p, prio, swap_map, cluster_info);
> + setup_swap_info(p, prio, swap_map, cluster_info);
> + spin_unlock(&p->lock);
> + spin_unlock(&swap_lock);
> + /*
> + * Guarantee swap_map, cluster_info, etc. fields are used
> + * between get/put_swap_device() only if SWP_VALID bit is set
> + */
> + stop_machine(swap_onoff_stop, NULL, cpu_online_mask);
> + spin_lock(&swap_lock);
> + spin_lock(&p->lock);
> + _enable_swap_info(p);
> spin_unlock(&p->lock);
> spin_unlock(&swap_lock);
> }
> @@ -2395,7 +2482,8 @@ static void reinsert_swap_info(struct swap_info_struct *p)
> {
> spin_lock(&swap_lock);
> spin_lock(&p->lock);
> - _enable_swap_info(p, p->prio, p->swap_map, p->cluster_info);
> + setup_swap_info(p, p->prio, p->swap_map, p->cluster_info);
> + _enable_swap_info(p);
> spin_unlock(&p->lock);
> spin_unlock(&swap_lock);
> }
> @@ -2498,6 +2586,17 @@ 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
> + */
> + stop_machine(swap_onoff_stop, NULL, cpu_online_mask);
> +
> flush_work(&p->discard_work);
>
> destroy_swap_extents(p);
> @@ -3263,17 +3362,11 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
> unsigned char has_cache;
> int err = -EINVAL;
>
> - if (non_swap_entry(entry))
> - goto out;
> -
> - p = swp_swap_info(entry);
> + p = get_swap_device(entry);
> if (!p)
> - goto bad_file;
> -
> - offset = swp_offset(entry);
> - if (unlikely(offset >= p->max))
> goto out;
>
> + offset = swp_offset(entry);
> ci = lock_cluster_or_swap_info(p, offset);
>
> count = p->swap_map[offset];
> @@ -3319,11 +3412,9 @@ 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;
> }
>
> /*
> @@ -3415,6 +3506,7 @@ 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
> @@ -3422,15 +3514,15 @@ 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);
>
> @@ -3448,9 +3540,8 @@ 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;
> }
>
> /*
> @@ -3502,10 +3593,11 @@ 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;
> }
>
> /*
> --
> 2.20.1
>

--
Michal Hocko
SUSE Labs

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