On 07/04/2024 07:54, Huang Ying wrote: > During reviewing a patch to fix the race condition between > free_swap_and_cache() and swapoff() [1], it was found that the > document about how to prevent racing with swapoff isn't clear enough. > Especially RCU read lock can prevent swapoff from freeing data > structures. So, the document is added as comments. > > [1] https://lore.kernel.org/linux-mm/c8fe62d0-78b8-527a-5bef-ee663ccdc37a@xxxxxxxxxx/ > > Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx> > Cc: Ryan Roberts <ryan.roberts@xxxxxxx> > Cc: David Hildenbrand <david@xxxxxxxxxx> > Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx> > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > Cc: Minchan Kim <minchan@xxxxxxxxxx> LGTM! Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx> > --- > mm/swapfile.c | 26 +++++++++++++------------- > 1 file changed, 13 insertions(+), 13 deletions(-) > > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 4919423cce76..6925462406fa 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -1226,16 +1226,15 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p, > > /* > * When we get a swap entry, if there aren't some other ways to > - * prevent swapoff, such as the folio in swap cache is locked, page > - * table lock is held, etc., the swap entry may become invalid because > - * of swapoff. Then, we need to enclose all swap related functions > - * with get_swap_device() and put_swap_device(), unless the swap > - * functions call get/put_swap_device() by themselves. > + * prevent swapoff, such as the folio in swap cache is locked, RCU > + * reader side is locked, etc., the swap entry may become invalid > + * because of swapoff. Then, we need to enclose all swap related > + * functions with get_swap_device() and put_swap_device(), unless the > + * swap functions call get/put_swap_device() by themselves. > * > - * Note that when only holding the PTL, swapoff might succeed immediately > - * after freeing a swap entry. Therefore, immediately after > - * __swap_entry_free(), the swap info might become stale and should not > - * be touched without a prior get_swap_device(). > + * RCU reader side lock (including any spinlock) is sufficient to > + * prevent swapoff, because synchronize_rcu() is called in swapoff() > + * before freeing data structures. > * > * Check whether swap entry is valid in the swap device. If so, > * return pointer to swap_info_struct, and keep the swap entry valid > @@ -2495,10 +2494,11 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile) > > /* > * Wait for swap operations protected by get/put_swap_device() > - * to complete. > - * > - * We need synchronize_rcu() here to protect the accessing to > - * the swap cache data structure. > + * to complete. Because of synchronize_rcu() here, all swap > + * operations protected by RCU reader side lock (including any > + * spinlock) will be waited too. This makes it easy to > + * prevent folio_test_swapcache() and the following swap cache > + * operations from racing with swapoff. > */ > percpu_ref_kill(&p->users); > synchronize_rcu();
