2021-04-20 13:34:52

by Miaohe Lin

Subject: [PATCH v3 4/4] mm/shmem: fix shmem_swapin() race with swapoff

When I was investigating the swap code, I found the below possible race
window:

CPU 1 CPU 2
----- -----
shmem_swapin
swap_cluster_readahead
if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
swapoff
percpu_ref_kill(&p->users)
synchronize_rcu()
wait_for_completion
..
si->swap_file = NULL;
struct inode *inode = si->swap_file->f_mapping->host;[oops!]

Close this race window by using get/put_swap_device() to guard against
concurrent swapoff.

Fixes: 8fd2e0b505d1 ("mm: swap: check if swap backing device is congested or not")
Signed-off-by: Miaohe Lin <[email protected]>
---
mm/shmem.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 26c76b13ad23..936ba5595297 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1492,15 +1492,21 @@ static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
struct shmem_inode_info *info, pgoff_t index)
{
+ struct swap_info_struct *si;
struct vm_area_struct pvma;
struct page *page;
struct vm_fault vmf = {
.vma = &pvma,
};

+ /* Prevent swapoff from happening to us. */
+ si = get_swap_device(swap);
+ if (unlikely(!si))
+ return NULL;
shmem_pseudo_vma_init(&pvma, info, index);
page = swap_cluster_readahead(swap, gfp, &vmf);
shmem_pseudo_vma_destroy(&pvma);
+ put_swap_device(si);

return page;
}
--
2.19.1

2021-04-21 01:04:42

by Huang, Ying

Subject: Re: [PATCH v3 4/4] mm/shmem: fix shmem_swapin() race with swapoff

Miaohe Lin <[email protected]> writes:

> When I was investigating the swap code, I found the below possible race
> window:
>
> CPU 1 CPU 2
> ----- -----
> shmem_swapin
> swap_cluster_readahead
> if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
> swapoff
> percpu_ref_kill(&p->users)
> synchronize_rcu()
> wait_for_completion

I don't think the above 3 lines are relevant for the race.

> ..
> si->swap_file = NULL;
> struct inode *inode = si->swap_file->f_mapping->host;[oops!]
>
> Close this race window by using get/put_swap_device() to guard against
> concurrent swapoff.
>
> Fixes: 8fd2e0b505d1 ("mm: swap: check if swap backing device is congested or not")
> Signed-off-by: Miaohe Lin <[email protected]>
> ---
> mm/shmem.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 26c76b13ad23..936ba5595297 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1492,15 +1492,21 @@ static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
> static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
> struct shmem_inode_info *info, pgoff_t index)
> {
> + struct swap_info_struct *si;
> struct vm_area_struct pvma;
> struct page *page;
> struct vm_fault vmf = {
> .vma = &pvma,
> };
>
> + /* Prevent swapoff from happening to us. */
> + si = get_swap_device(swap);

Better to put get/put_swap_device() in shmem_swapin_page(), that make it
possible for us to remove get/put_swap_device() in lookup_swap_cache().

Best Regards,
Huang, Ying

> + if (unlikely(!si))
> + return NULL;
> shmem_pseudo_vma_init(&pvma, info, index);
> page = swap_cluster_readahead(swap, gfp, &vmf);
> shmem_pseudo_vma_destroy(&pvma);
> + put_swap_device(si);
>
> return page;
> }