When I was investigating the swap code, I found the below possible race
window:
CPU 1 CPU 2
----- -----
do_swap_page
synchronous swap_readpage
alloc_page_vma
swapoff
release swap_file, bdev, or ...
swap_readpage
check sis->flags is ok
access swap_file, bdev...[oops!]
si->flags = 0
Using current get/put_swap_device() to guard against concurrent swapoff for
swap_readpage() looks terrible because swap_readpage() may take really long
time. And this race may not be really pernicious because swapoff is usually
done when system shutdown only. To reduce the performance overhead on the
hot-path as much as possible, it appears we can use the percpu_ref to close
this race window(as suggested by Huang, Ying).
Fixes: 235b62176712 ("mm/swap: add cluster lock")
Signed-off-by: Miaohe Lin <linmia...@huawei.com>
---
include/linux/swap.h | 2 +-
mm/memory.c | 10 ++++++++++
mm/swapfile.c | 28 +++++++++++-----------------
3 files changed, 22 insertions(+), 18 deletions(-)
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 849ba5265c11..9066addb57fd 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -513,7 +513,7 @@ sector_t swap_page_sector(struct page *page);
static inline void put_swap_device(struct swap_info_struct *si)
{
- rcu_read_unlock();
+ percpu_ref_put(&si->users);
}
#else /* CONFIG_SWAP */
diff --git a/mm/memory.c b/mm/memory.c
index cc71a445c76c..8543c47b955c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3311,6 +3311,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
{
struct vm_area_struct *vma = vmf->vma;
struct page *page = NULL, *swapcache;
+ struct swap_info_struct *si = NULL;
swp_entry_t entry;
pte_t pte;
int locked;
@@ -3339,6 +3340,11 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
}
+ si = get_swap_device(entry);
+ /* In case we raced with swapoff. */
+ if (unlikely(!si))
+ goto out;
+
delayacct_set_flag(DELAYACCT_PF_SWAPIN);
page = lookup_swap_cache(entry, vma, vmf->address);
swapcache = page;
@@ -3514,6 +3520,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
unlock:
pte_unmap_unlock(vmf->pte, vmf->ptl);
out:
+ if (si)
+ put_swap_device(si);
return ret;
out_nomap:
pte_unmap_unlock(vmf->pte, vmf->ptl);
@@ -3525,6 +3533,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
unlock_page(swapcache);
put_page(swapcache);
}
+ if (si)
+ put_swap_device(si);
return ret;
}
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 724173cd7d0c..01032c72ceae 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1280,18 +1280,12 @@ static unsigned char __swap_entry_free_locked(struct
swap_info_struct *p,
* via preventing the swap device from being swapoff, until
* put_swap_device() is called. Otherwise return NULL.
*
- * The entirety of the RCU read critical section must come before the
- * return from or after the call to synchronize_rcu() in
- * enable_swap_info() or swapoff(). So if "si->flags & SWP_VALID" is
- * true, the si->map, si->cluster_info, etc. must be valid in the
- * critical section.
- *
* Notice that swapoff or swapoff+swapon can still happen before the
- * rcu_read_lock() in get_swap_device() or after the rcu_read_unlock()
- * 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.
+ * percpu_ref_tryget_live() in get_swap_device() or after the
+ * percpu_ref_put() 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()
@@ -1319,21 +1313,21 @@ struct swap_info_struct *get_swap_device(swp_entry_t
entry)
si = swp_swap_info(entry);
if (!si)
goto bad_nofile;
-
- rcu_read_lock();
if (data_race(!(si->flags & SWP_VALID)))
- goto unlock_out;
+ goto out;
+ if (!percpu_ref_tryget_live(&si->users))
+ goto out;
offset = swp_offset(entry);
if (offset >= si->max)
- goto unlock_out;
+ goto put_out;
return si;
bad_nofile:
pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
out:
return NULL;
-unlock_out:
- rcu_read_unlock();
+put_out:
+ percpu_ref_put(&si->users);
return NULL;
}
--
2.19.1
