Re: [PATCH] mm/swapfile: fix data races in try_to_unuse()

[Wei Yang]

On Mon, Feb 24, 2020 at 04:15:03PM -0500, Qian Cai wrote:
>si->inuse_pages could be accessed concurrently as noticed by KCSAN,
>
> write to 0xffff98b00ebd04dc of 4 bytes by task 82262 on cpu 92:
>  swap_range_free+0xbe/0x230
>  swap_range_free at mm/swapfile.c:719
>  swapcache_free_entries+0x1be/0x250
>  free_swap_slot+0x1c8/0x220
>  __swap_entry_free.constprop.19+0xa3/0xb0
>  free_swap_and_cache+0x53/0xa0
>  unmap_page_range+0x7e0/0x1ce0
>  unmap_single_vma+0xcd/0x170
>  unmap_vmas+0x18b/0x220
>  exit_mmap+0xee/0x220
>  mmput+0xe7/0x240
>  do_exit+0x598/0xfd0
>  do_group_exit+0x8b/0x180
>  get_signal+0x293/0x13d0
>  do_signal+0x37/0x5d0
>  prepare_exit_to_usermode+0x1b7/0x2c0
>  ret_from_intr+0x32/0x42
>
> read to 0xffff98b00ebd04dc of 4 bytes by task 82499 on cpu 46:
>  try_to_unuse+0x86b/0xc80
>  try_to_unuse at mm/swapfile.c:2185
>  __x64_sys_swapoff+0x372/0xd40
>  do_syscall_64+0x91/0xb05
>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
>The plain reads in try_to_unuse() are outside si->lock critical section
>which result in data races that could be dangerous to be used in a loop.
>Fix them by adding READ_ONCE().
>
>Signed-off-by: Qian Cai <cai@xxxxxx>
>---
> mm/swapfile.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
>diff --git a/mm/swapfile.c b/mm/swapfile.c
>index a65622eec66f..36fd1536a83d 100644
>--- a/mm/swapfile.c
>+++ b/mm/swapfile.c
>@@ -2137,7 +2137,7 @@ int try_to_unuse(unsigned int type, bool frontswap,
> 	swp_entry_t entry;
> 	unsigned int i;
> 
>-	if (!si->inuse_pages)
>+	if (!READ_ONCE(si->inuse_pages))
> 		return 0;
> 
> 	if (!frontswap)
>@@ -2153,7 +2153,7 @@ int try_to_unuse(unsigned int type, bool frontswap,
> 
> 	spin_lock(&mmlist_lock);
> 	p = &init_mm.mmlist;
>-	while (si->inuse_pages &&
>+	while (READ_ONCE(si->inuse_pages) &&

The change is not wrong. But since it is not protected by the lock, some
status in swap_info_struct could still be modified after we test this
inuse_pages is not zero. Would this be some problem?


-- 
Wei Yang
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