| Message ID | 20210420133048.6773-3-linmiaohe@huawei.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | close various race windows for swap | expand |
Miaohe Lin <linmiaohe@huawei.com> writes: > When I was investigating the swap code, I found the below possible race > window: > > CPU 1 CPU 2 > ----- ----- > do_swap_page > if (data_race(si->flags & SWP_SYNCHRONOUS_IO) > swap_readpage > if (data_race(sis->flags & SWP_FS_OPS)) { > swapoff > p->flags &= ~SWP_VALID; > .. > synchronize_rcu(); > .. You have deleted SWP_VALID and RCU solution in 1/4, so please revise this. > p->swap_file = NULL; > struct file *swap_file = sis->swap_file; > struct address_space *mapping = swap_file->f_mapping;[oops!] > > Note that for the pages that are swapped in through swap cache, this isn't > an issue. Because the page is locked, and the swap entry will be marked > with SWAP_HAS_CACHE, so swapoff() can not proceed until the page has been > unlocked. > > 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). This needs to be revised too. Unless you squash 1/4 and 2/4. > Fixes: 0bcac06f27d7 ("mm,swap: skip swapcache for swapin of synchronous device") > Reported-by: kernel test robot <lkp@intel.com> (auto build test ERROR) > Signed-off-by: Miaohe Lin <linmiaohe@huawei.com> > --- > include/linux/swap.h | 9 +++++++++ > mm/memory.c | 9 +++++++++ > 2 files changed, 18 insertions(+) > > diff --git a/include/linux/swap.h b/include/linux/swap.h > index c9e7fea10b83..46d51d058d05 100644 > --- a/include/linux/swap.h > +++ b/include/linux/swap.h > @@ -527,6 +527,15 @@ static inline struct swap_info_struct *swp_swap_info(swp_entry_t entry) > return NULL; > } > > +static inline struct swap_info_struct *get_swap_device(swp_entry_t entry) > +{ > + return NULL; > +} > + > +static inline void put_swap_device(struct swap_info_struct *si) > +{ > +} > + > #define swap_address_space(entry) (NULL) > #define get_nr_swap_pages() 0L > #define total_swap_pages 0L > diff --git a/mm/memory.c b/mm/memory.c > index 27014c3bde9f..7a2fe12cf641 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; > @@ -3338,6 +3339,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > goto out; > } > > + /* Prevent swapoff from happening to us. */ > + si = get_swap_device(entry); There's struct swap_info_struct *si = swp_swap_info(entry); in do_swap_page(), you can remove that. Best Regards, Huang, Ying > + if (unlikely(!si)) > + goto out; > > delayacct_set_flag(current, DELAYACCT_PF_SWAPIN); > page = lookup_swap_cache(entry, vma, vmf->address); > @@ -3514,6 +3519,8 @@ 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 +3532,8 @@ 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/include/linux/swap.h b/include/linux/swap.h index c9e7fea10b83..46d51d058d05 100644 --- a/include/linux/swap.h +++ b/include/linux/swap.h @@ -527,6 +527,15 @@ static inline struct swap_info_struct *swp_swap_info(swp_entry_t entry) return NULL; } +static inline struct swap_info_struct *get_swap_device(swp_entry_t entry) +{ + return NULL; +} + +static inline void put_swap_device(struct swap_info_struct *si) +{ +} + #define swap_address_space(entry) (NULL) #define get_nr_swap_pages() 0L #define total_swap_pages 0L diff --git a/mm/memory.c b/mm/memory.c index 27014c3bde9f..7a2fe12cf641 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; @@ -3338,6 +3339,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) goto out; } + /* Prevent swapoff from happening to us. */ + si = get_swap_device(entry); + if (unlikely(!si)) + goto out; delayacct_set_flag(current, DELAYACCT_PF_SWAPIN); page = lookup_swap_cache(entry, vma, vmf->address); @@ -3514,6 +3519,8 @@ 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 +3532,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) unlock_page(swapcache); put_page(swapcache); } + if (si) + put_swap_device(si); return ret; }
When I was investigating the swap code, I found the below possible race window: CPU 1 CPU 2 ----- ----- do_swap_page if (data_race(si->flags & SWP_SYNCHRONOUS_IO) swap_readpage if (data_race(sis->flags & SWP_FS_OPS)) { swapoff p->flags &= ~SWP_VALID; .. synchronize_rcu(); .. p->swap_file = NULL; struct file *swap_file = sis->swap_file; struct address_space *mapping = swap_file->f_mapping;[oops!] Note that for the pages that are swapped in through swap cache, this isn't an issue. Because the page is locked, and the swap entry will be marked with SWAP_HAS_CACHE, so swapoff() can not proceed until the page has been unlocked. 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: 0bcac06f27d7 ("mm,swap: skip swapcache for swapin of synchronous device") Reported-by: kernel test robot <lkp@intel.com> (auto build test ERROR) Signed-off-by: Miaohe Lin <linmiaohe@huawei.com> --- include/linux/swap.h | 9 +++++++++ mm/memory.c | 9 +++++++++ 2 files changed, 18 insertions(+)